It's not that it's bad, it's just not the first choice for gamers anymore.
It can still be an impressive value for the money if including it is a stretch for your budget, but not too far above it (in cost) is the Haswell entry level i5. Because the i5 can process more instructions per clock cycle, is generally viewed as better for games, and so the FX takes a bad wrap. The FX is a great workstation though (editing/rendering), and if you game at 1080p and also multitask (record, stream, chat on second monitor while gaming), it can have some advantages for the money.
I have one myself, and it does just fine with GTA V and Witcher 3.....but the Intel setup with the same GPU/resolution gets more FPS. Basically all depends on the money you can spend, and how you balance that with your GPU budget and performance expectations.