Currently they are far less useful than the fighters guild or mages guild.
Could you consider adding some relevant skills to these supposed skill lines please.
For example:
Dark brotherhood
Agonising death - poison DoT
Silent blade - attack that ignores armour
Unerring accuracy - 15% weapon crit stacks with savagery
Ultimate - Night mothers kiss - big poison DoT
Thieves guild
Backstab - when behind target do 15% extra dmg for x secs
Quick hands
25% chance opponent misses due to physical attack being redirected
Escaping trap
On trigger grants minor expedition to user and snares opponent
Come on ZOS give this skill lines some parity with the big two...!