I didn't mind uldir but I didn't go far into mythic either. Ghuun was definitely a bit hard but seemed largely fair, at least on heroic.
My biggest gripe with G'huun, at all difficulty levels, is that he's largely a gimmick boss, which is not something you should really have for an end boss of a raid. At least, not to such an extent with a more experimental mechanic. (About 70% of the fight time I think is doing the gimmick, with the final 30% being more standard gameplay which is also arguably harder.) So you have to do a bunch of tedious gimmick work before getting to the meat of the fight and while doing boring stuff first before the engaging hard part isn't new, the orb dunking mechanic wasn't exactly difficult with proper coordination beforehand for more competent groups.
If you want to experiment with new mechanic ideas, I think it's best to have them at other points in a longer raid besides the final boss. 1-3 boss raids don't quite have as much weight for this rule.
I think one of the last times there was a similar or potentially worse end boss for a tier of raiding was Malygos capping off Tier 7 in Wrath with uh...a shitty vehicle fight. Cause of course they wanted to shoehorn in their new tech toy! Although that's maybe a bit more bearable with the shitty part being the end of the fight, rather than at the beginning or in the middle. The flying disc part was pretty cool actually, and as a DK, I liked being needed to grip sparks to position them properly (GRIPS STILL OP AS ALL HELL even today.)
Kil'Jaeden introduced the darkness shroud mechanic, which was pretty successful, though the fight in general was hard as balls before it was nerfed, and even after it was. Though that whole tier in Tomb of Sargeras was much harder than many other raids.
Archimonde was pretty great, but long. Garrosh was pretty great, but long. I would say Argus was pretty good. etc. (those happen to be expansion-ending bosses though, rather than tier ending bosses). Nighthold Gul'dan was long, and the Mythic final phase made the boss that much more engaging because the first parts of the fight were still fairly difficult, but you had to execute them consistently to make it to the Illidan shadow demon, which was completely different (only similarity was the encounter arena basically) and required a whole new set of tactics you had to learn or create. One of the best fights in the game, and that's how ALL important end-bosses should be, at least on Mythic. Though not every raid needs to have secret or extra phases like that -- see Helya in Trial of Valor, for example or Lei Shen in Throne of Thunder I think (ignoring Ra-Den as an additional boss).
Also, although G'huun is an old god experiment/minion/whatever, he doesn't quite have the same schtick as the other REAL old gods we've defeated in the past like C'thuun and Yogg-Saron. Maybe you can count Garrosh as an extension of Y'shaarj a bit. The old god fights typically fuck with your mind more with whispers, and porting you into different spaces (whether in your head or into C'thuun's stomach).
So I think it just wasn't a very good fight for an end boss mechanically, and didn't capture the essence of makes most old god-related fights more memorable.
You could probably do a run down analysis of all the final raid bosses in the game and compare what makes them stand out mechanically and thematically, cross referenced with people's opinions about them. Certainly would be easier to limit the scope to just end-of-raid final bosses, rather than including all bosses. A different analysis could be run on introductory bosses, and mid-tier bosses as well.