The term Tang-Dao has two different, but related meanings.
The first is as the original name of the two races (at the time one race) the Brorg-Dao and the Tsung-Dao.
This was at a time of great instability, where their violent and destructive society was threatening to crumble around them.
The Brorg-Dao broke away and formed their own society, while those that remained eventually changed their course and became the Tsung-dao.
The details of Tang-Dao society are some what sketchy, obscured through the passage of time.
Both the Brorg and Tsung-Dao record them as barbaric, backward and ignorant. They are said to have become increasingly obsessed with the worship of blood thirsty gods through extremely violent religions. The most significant of these gods was Tang-Roth (now considered a demon by the Brorg and Tsung-Dao.)
Their culture is believed to have become self destructive, with rituals and sacrifices valued more than practicalities. Fortunately, they abandoned this life style before it was too late.

The modern definition of the Tang-Dao refers to the followers of Lon-Pa, the Heretic.
They use this title because they consider themselves a revival of their peoples former glory.
Their movement is based on the idea that their people used to be strong and respected, but weak leaders and compromise with foreigners has left them weak and disgraced.
The historical accuracy of a lot of their claims are extremely questionable, as are the perceived slights they feel their nation has suffered.
They have various cells hidden through out the Tsung-Dao lands, waging a gorilla war against the authorities. They work mostly as bandits, raiding and robbing travelers. This allows them to hit at Tsung-Dao society and also fund their own organisation.
Their organisation is based on encouraging a sense that they are victims and that they need to reclaim some sort of past golden age. The way to do this is to remove their corrupt and weak leaders and ineffectual rules of conduct that only hold them back from their true power.
In troubled and uncertain times, these sort of simple message can be quite powerful and many young and impressionable Tsung-Dao have been won over to this cause.
The Heretics world view rejects a lot of aspects of Tsung-Dao culture, which he considers signs of weakness and symptoms of a lack of pride in their culture.
The restrained, reserved nature of the Tsung-Dao and their ordered way of life is entirely discarded by his disciples.
Instead the Heretic encourages his followers to take what they want, through strength of will, threats and if necessary violence.
Tsung-Dao thrive on order, direction and rules, so when these concepts are turned on their heads it leads them to becoming dangerously violent and aggressive.
The Tang-Dao laugh at the Tsung-Dao as repressed and timid, but they lack the insight to see how unharmonious their own existence has become.
They struggle to restrain their emotions, lashing out both verbally and physically at each other. They are always eager for battle, as it gives them something external to focus their inner turmoil on.
The one thing about their beliefs that is fairly authentic to the original Tang-Dao is their frightening war masks. With pale faces and bright manes of hair, these help strike fear into their enemies hearts. They dress in a different style of armour to Tsung-Dao, though this is based on the preference of the Heretics second in command Reng-Lo to model her troops after her own armour, rather than anything historical.
Interestingly they also relax the Tsung-Dao modesty rules, often bearing their arms or legs. This is scene as a sign of their deviancy by main stream Tsung-Dao society. The violent excesses, torture and rapes they have committed against travelers only reinforce this view of them as wild, immoral savages in the minds of the Tsung-Dao.
The irony of their organisation is that, for all their xenophobic rhetoric and talk of national pride, due to its relatively low numbers they receive a great deal of foreign help.
Mercenary Junlocks are their most common helper (kept in line with loot captured by their raiding parties or The Speakers powers of persuasion) Though they have also been know to get direct help from the Nameless.
Though this hypocracy may seem obvious to outsiders, the Heretics followers are so engrossed in his cult that they over look such clear inconsistencies between it's doctrine and it's reality.
For his part, The Speaker considers the movement useful pawns, but nothing more. Any concept of a Tsung-Dao revival that they wish to bring about is meaningless to him, and would actually go against his ultimate goal of the West's total domination of Harkovast.
The Tang-Dao are just another method through which he hopes to spread discord and dissent, further weakening his enemies,
for the West has no allies,
only enemies and slaves.

So if the Tsung-Dao don't follow their rules and routines they can turn into drunken psychopaths with tunnel vision?

How much actual harm to the Tang-Dao cause in terms of bandit activity (are they widespread and successful enough to effect trade)? How about in their political goals, do they get anywhere with it, or are they just fighting against the odds for a hopeless cause?

As we see in the comic, bands of Tang-Dao are big enough (with bandit and mercenary support) to threaten whole towns.
It is The Tang-Dao's belief that if they could take over a few towns and score some major victories that more people would flock to their cause.
In terms of what they would do if they ever got power, they're goals are all pretty short sighted and beyond “putting things back the way they ought to be” its all very general in terms of policy.
The most likely outcome would be the Heretic being put in charge and them then using force to make everyone else obey them.
One thing they have in come with the Tang-Dao of old is that they would probably just run things into the ground.

Tsung-Dao thrive on structure and dont function well without it.
I should point out that they are not barely suppressed psychopaths though!
Being ordered and dutiful comes naturally to a Tsung-Dao, so to not behave this way, rather than being liberating or empowering as the Heretic claims, actually just makes them emotionally confused and agitated (hence the behaviour of his followers.)

Seriously Renard, you are like a ninja on this forum! You post so fast it makes my head spin.

The first thing I do when I get home is check the BBC, my DeviantArt account, and the forums here.

Although they do call me the Steel Ninja at work…

It's nice to know people find all this stuff interesting. I always assumed when I started the forum no one would bother to read it.

have the tang-dao taken full control over any land, or towns, or are they still just marauding around, with no land to semi-formally call their own?

They don't control any territory yet (hence them hiding out amid ruins.)
Obviously, if the Heretics plan works out then they'll get hold of some real estate!

Even if they get ahold of land, it remains to be seen if the Tang Dao can successfully control it and make anything of thier newly accquired territories. And given that the Tang-Dao are a bunch of nutters with agression and self control issues being lead by a manipulative, slef-justifying and murderous toad, it doesn't seem likely they could do much of anything productive in the long run.

Vendetto you make a legitimate point about the viability of a Tang-Dao state.
Of course, The Speaker doesn't care one jot about that as long as they continue to weaken his enemies and serve his purposes.

Don't discount the power of nationalism to make a broken system work. It helped keep france from being conquered during it's revolution, though didn't save it from the reign of terror. I doubt their rule would be french revolution like tho, the methods of revolution are different. perhaps the tang-dao rule would be more russian october revolution like. Small minority, with vast nationalism, claiming to fight for the benefit of all, but has the support of few, and takes over by gunpoint. Bolshevik actually means “majority” they went by that name to make it seem like they had the majority support (spoiler; they didn't).
Their rule will probably be characterized by a rapid drop in popularity, unrest, rule by fear (whether it's fear of foreign threats, or of the regime itself), and a truly dreadful foreign policy (might even bring about an all time low of good will towards the lands of the tsung/tang-dao).
most of this would be if they got more then one city of course. with just one city, it would probably just be marshal law 24/7,
.

Also it would depend on exactly how their revolution fits into the West's designs.
Since The Speaker is pulling the strings behind the scenes, what really matters is what he wants any state they form to do, more than what they want it to do.

More than likely it would suit the Speaker because the Tsung-Dao lands have a coastline that would be useful for landing and rallying troops, not to mention the toppling of the Tsung-Dao government would mean one less nation to put up large scale opposition to the West's armies.

A revolution would be favorable in most cases, unless the new leaders forget their old (and extremely dangerous) allies.

Renard sounds a pretty accurate picture of what The Speaker is looking for.
Ultimately everyone is expendable to him, and ultimately he is pretty much required to conquer or destroy all peoples.