Could be small or big.
My answer has always been that, Linux can't handle everything I'd ask out of it that I normally can with Windows. I know the games issue has been progressing far from the days when that used to have been an archaic flaw with Linux for the longest time. Games might not be the issue except for some concerns I have for some games.
I was taking some time a few moments ago, to check if a program called Firestorm Viewer would work on Linux Mint which could've been my distro of choice. And the description written on the linux page described exactly the kind of concerns I'd have for compatibility and usability from going Windows to Linux.
They said that their viewer was tested and designed to function mostly with Ubuntu and while it could work with other distros, it's not to be expected to be smooth.
That's the kind of sentiment and concern I have always had with Linux if I were to go from Windows to it. There are programs and tools on Windows that I have that are used for specific purposes and I know they will not function on Linux. Furthermore, incase anything breaks down, any and all solutions would only be applicable to that thing that would be far easier to solve than just being SOL if I was on Linux.
It is something as a user that I just can't simply afford to deal with on a regular basis if I made the switch.
So while I may not have too much of an issue running games, I won't have too much of an issue using alternatives, I won't have to deal with the Windows ecosystem .etc I will just be running into other walls that would simply make me second guess my decision and make me regret switching to the point where I would dip back into Windows in a hurry.
I had way more problems with kubuntu than manjaro by the way. Trying to remember back, I think I switched away from it because of some security issue that was a meme at that point? I was thinking of switching back but I merged the laptop's ssd and hdd in the disk manager so now I'd have to have a clean start and I got lazy.
I'm no longer in a situation where the laptop is especially useful to me and it was my trial run for deciding if I would switch on my desktop.
Rocket league and this
I could probably run a VM for the latter if I were on a desert island I guess
Yeah, Kubuntu has some issues. If you didn't have any trouble with Manjaro, CachyOS is similar, but without the BS, super fast and gaming focused. I'm using it now, and I was fully expecting to break it but it just keeps trucking.
Also, you can run both of those games on Linux. Steam's Proton tool is magic. ProtonDB users have Rocket League working on the Steam Deck. You probably just need the Heroic Games Launcher to get it from Epic. I don't have any personal experience with Rocket League, but Civ IV definitely runs through Steam with Proton, and it's not hard to mod games on there.