Linux

TL;DR: Basically gaming compatibility and additional complications, on top of all what is new due to Linux.

I have a brother trying to convince him to use Linux over Windows (or at least dual boot). I could make him use Manjaro (back then when I was using Manjaro myself) on a laptop. That was his first experience and he is a gamer who likes multiplayer games. So the experience was a bit mixed. Later I borrowed him my Steam Deck for 2 weeks and it was a torture to myself, as it was the launch period of the hardware. And then I convinced him to buy Steam Deck instead a laptop.

He still loves the Steam Deck and uses it here and there, especially on vacation. But as lot of primary multiplayer games he play do not work on Linux and because of complications with some non Steam games and lot of applications he had, such as Discord, he went back to Windows on his new PC. Some complications arised because of the Steam Deck and its limitations, but that did not change the fact how games he plays are not working.

But he admits that SteamOS is the better operating system. And he understands why it is what it is, but as said, that does not change the fact he cannot play some of his favorite games on Linux. But that is not all. You have to understand that newcomers who experience LInux for the first time, and switched reluctant without research, don't know what Wayland is, don't know differences between desktop environments and has to deal with compatibility layers on top of all other new Linux stuff for them.

Why your sister felt she has less control is just a feeling, because she know less, therefore can control less. It makes sense from her perspective, so I would not say its entirely wrong.