There are some games that don't work or don't work as well. Some anti-cheat systems don't work but the website protondb.com will tell you how compatible specific games are. For some people that's a deal breaker and that's ok, hopefully as adoption increases the situation will get better. I was disappointed to find out that vermintide 2 doesn't work for example.

I have no idea on the pirating side.

Most of my games work right out of the box, and the ones that have problems are ones that I'd also have to fiddle with for more than a 1-minute check to ProtonDB are ones I'd have to fiddle with on Windows. However I also do not touch anything with online multiplayer or anticheat, and I know games with kernel-level anticheat tend to not handle Linux well on anything but a Steam Deck.

I swapped a PC I had mostly for gaming over to Linux. I'm having a pretty nice time.

As for piracy, I know pirated games that need to be emulated because they are originally Nintendo Switch games or something work well. No idea for pirated PC games.

If you use Steam quite a bit, check out the ProtonDB Web site. That can tell you the level of compatibility. 90% of my library seemed to be covered, and it's seamless. I was impressed!

Edit to say: One problem I had was getting my Brother printer to work over WiFi. That was some annoying arcane wizardry, but I finally got it to work.