A 3090 or 4090 can easily pull down enough power that most consumer UPSes (besides the larger tower ones) will do their 'beep of overload', which at best is annoying, at worst causes stability issues.
I think there's a sweet spot around 180-250W for these cards, unless you _really_ need top-end performance.
To me it's the PCI lanes that are the issue. Chances of a random gamer having a PSU that can run dual cards is excellent...chances of dual x16 electrical not so much.
I tried dual in x16 x4 and inference performance cratered versus a single
I think there's a sweet spot around 180-250W for these cards, unless you _really_ need top-end performance.