From my limited experience with fluid/flexible/elastic web design and just general experience with graphic design and how minute size changes can throw whole layouts off, I’d say in the context of games it’s probably a huge headache. Keeping things tidy visually is one thing, but increasing the size of text will increase the space needed for descriptions which in some cases will necessitate scrolling and additional inputs. If you need to compare things then screen space becomes an issue. Some UIs are very tight/rigid before increasing the size of certain elements so that’s another potential problem with no easy fix.
I think with the Switch and soon the Steam Deck, decent scaling will steadily become more widespread but it doesn’t seem to me like an easy thing to do retrospectively. I’m not a developer though!! :-)