I've only just find it hard to find fault with this series, though I've just finished watching it, and if I really thought about it I might, but why would I search it out?
Exceptional writing and direction provide an unpredictable plot (well, mostly) extremely well paced. Acting is outstanding from all present. Characters are mostly pretty well rounded and not simply caricatures. Evil walks in human form and retains an element of mystery even until the end.
It IS very gory, and if you don't like realistic fights then don't watch this. That said, one flaw, perhaps, might be the way characters can sustain grievous injuries and still hit their opponent just as hard with apparently no weakening. That's a piece of nonsense, but something of a trope so can be forgiven. Fights are forceful and balletic.
The fact that these events are awful is as they should be. This world is not a nice place to stroll : it is not pretty, it is not gentle, it is not clean, but it manages, just, to be human.