would you say the teagues were an impressive or unipressive dynasty, given how long they lasted, but never managed to establish themselves as the true kings of the rivers and hills?

Given how little info we have, it’s hard to tell. 

However, I will venture a guess that the Teagues were the least stable of the Riverlander dynasties. Elio describes them as “Torrence Teague and his heirs succeeded for a few generations, then lost it.” Most Riverlander dynasties ruled for periods of hundreds of years, the Teagues seem to have problems going longer than 60 years at a clip.