They don’t have a bad post imp, however they do have some terrible matchups, especially against strong cav archer civs if they ever get their comp rolling.
That said Bulgarians have nice bonuses in the early and midgame, krepost rushes are also underrated imo.
Against goths going 2h swordsmen with the UT will shut down any infantry play they have.
It will force them into a HC play, which you should be able to counter with Hussar that have strirrups, HC/Halb is too weak imo, as any siege or skirm play works.
Basically forcing Goths off Infantry is usually a winning strat against them, though imo they should never get there, Goths get an Ok dark age, a generic Feudal age and compared to many civs a horrible midgame castle age.
They hold till imp or die, but if they reach their goal they are a terrifying civ, definitely one of the strongest, though not unwinable, you want to prevent them getting there.