You played it, the GM needs to run the game for all of their players. That includes aquaman getting "plenty of meaningful moments" and disguising when they are a job for aquaman so nobody notices or mentions that the GM is doing just that. 2e style & PF2 style feat based variants mitigate these issues better than 5e's.
If a GM's players are making build decisions for these kind of reasons it sounds like they are running a wildly different game sporting a level of knock down dragout pvp than any I've ever heard of. Very few monsters cast wall/cube of force making it an extreme edge case.
No it's not just a warlock thing. Warlock was rarely if ever even considered for great multiclass builds back in 3.5 yet it too had pages & pages of great MC builds. The big difference though is that the GM had many more tools & dials they could massage to discretely adjust the relative power between players when needed.
I am really confused, why you bring up 3.5...
... I mean, if every broken build in 5e involves the warlock and eldritch blast, it seems that the warlock is the offender, not multiclass in general.
And going from your comments, your game is just about beating a big bag of hps... probably not, but it is a bit much to draw conclusions from a few statements about other people's games.