[Corinthian]

In my opinion there are three kinds of thugs.

1) Raiders: This is the most common and useful type. They rely on speed, flight, teleportation/CT and stealth to capture poorly defended provinces. They are of less use vs the computer because the AI tend to leave a similar number of chaff troops in each province. Humans tent to not waste money on defenses in non-critical provinces due to money restraints.

Examples include Glamour Cavalry, Shishis, Eagle kings.

2) The second kind of thug is the army thug. Typically a Herse or similarly tough recruitable commander. Give them a frostbrand or a demon whip and mix them in with normal elite troops for some extra sting. Also serves to protect normal troops from enemy thugs that use mistform/mirror image/body ethereal etc. Unlike most thugs they rely on the normal troops and their battle line for their survivability. So take care during placement.
In practice this takes a lot of micro so you rarely see this kind of thug.

3) The third kind is the Main Battle Thug. The Dominions answer to the main battle tank. Useful because they are spammable and affordable. They can make up entire armies of their own or fight together with real armies with some good scripting. When fighting with real armies they must not spend too much time buffing though, or the battle will be over before they can get to it.
The operative word here is cost effectiveness. They must be cheap or you might as well spend the gems on summonable troops.

Examples includes Shuras, Ujigamis, Skrattis, pre-CBM Zammzumites and again, Shishis. Most of these are nationals so you wont see them too often either.