What's wrong with SFV as a fighting game?
You already got a few answers but I didn't see anyone point this particular point.
SFV wasn't exactly "incredibly bad" - it was just really bland. It wasn't fun, all of the characters feel stiff and slow (even super agility characters that were super fast in SF3S/SFA). The main issue, as I see it at least, is that SF5 was a really bad release
at the time - meaning that Capcom desperately needed a slam dunk for SF5. The reason being is that the last "really good" game they had released at that point was in the
year of our lord 2000 (maybe 2002). SF4 released and was really fucking bland, MvC3 release and was pretty bland, they absolutely shit the bed with Street Fighter x Tekken (which had the obvious opportunity to be unbelievably hype) - they are
still in the middle of a
twenty year period of just releasing hot garbage for fighting games.
Not only were the games really bad to that point - there's also Capcom's awful business practices. Even on their unpopular and mediocre games - there was
tons of DLC, season passes, "new" versions, ports, remixes, and in some cases literal power upgrades that you can buy. There (after Street Figher 3) were
very few new characters - and the ones that were added were all kind of cheating (Laura from SF5 is just Sean from SF3's older sister, Kage is just "Eviler Ryu" and so on).
A lot of the SF community (hell, the FGC as a whole) were hoping that SF5 would be a "return to form" after 17 years of dogshit from Capcom. A lot of people hoped that a lot of the community feedback had gotten through to Capcom - about what players wanted out of a FG. It released and not only was it really bland and kind of awful (which was not what players wanted), but it was
clearly rushed out - there were huge bugs and virtually no content besides online mode, practice mode, and vs. computer mode. It was, for many people, a sign that Capcom was no longer a "serious" fighting game creator and potentially would never be for a very long time. It was the exact moment that a lot of people walked away from the Capcom Fighting Games division.
So SF5 is bad as a fighting game - but one of the reasons it's really hated (I think, at least) is that it's the game that made a lot of old SF fans think to themselves "maybe I just don't like Street Fighter - I haven't liked it in 15 years".