Usually games that require some shitty anti-cheat.

Games where the devs actively reject Linux support.

For my library of 390 games, only 2 are "Borked".

One is Magic: Duels, which was discontinued by WOTC back in 2019. It's still playable on Windows, but seeing as it was kind of a weird experiment they did that ultimately got replaced by Arena I can understand the lack of support there. Also it was free, and I got clean a few years ago, so I'm not salty.

The other of Flatout 3 (the car racing/destruction game, not to be confused with the similarly named Fallout 3). I remembered seeing ads and reviews for Flatout back in PSM when I was a kid. Never got to play it back then, but I grabbed the series bundle on sale at some point. Still haven't played any of them yet, but it appears the issue with Flatout 3 in particular is... It's a bad game. Just learned this now, but apparently it was made by a different developer than the first games and is, by review score, one of the worst games of all time. So there's probably not a whole lot of demand for it on Linux.

Fun stuff. Always neat to find a new way to look at the library.

Destiny 2 and Tarkov is my guess.