When Valveannounced the Steam Deckon Thursday, the company said that the platform “can pretty much run anything you could run on a PC.” Although that’s true in most cases, the “pretty much” qualifier could make a big difference. As it stands, popular games likeApex Legends, Destiny 2,andRainbow Six Siegewon’t work on the platform.

This comes down to an issue with SteamOS, a compatibility layer known as Proton, and anti-cheat software. SteamOS is based on Linux, which is a problem because the vast majority of games available on Steam are built to run on Windows. That’s where Proton comes in. Proton is a compatibility layer from Valve that allows Windows games to run on Linux.

Proton is available now as part of Steam Play, and many popular games work just fine with it. However, some big titles don’t. According toProtonDB, only three of the top 10most popular Steam gameshave native Linux support —Counter-Strike: Global Offensive, Dota 2,andTeam Fortress 2(all Valve-developed games).

Rust,meanwhile, has a “silver” rating, meaning it runs with minor issues, andGrand Theft Auto 5has a “gold” rating, meaning it runs after tweaks.PlayerUnknown’s Battlegrounds, Apex Legends, Destiny 2,andRainbow Six Siegeare all in the “borked” category, meaning they’re critically unplayable.

The reason is anti-cheat software. This software has some issues with Linux and the Proton compatibility layer, which has beena problem for yearsfor Linux gamers. The Steam Deck is bringing the issue center stage.

Most games that use some sort of anti-cheat software will either not work or will carry some significant problems. Easy Anti-Cheat, one of the most popular anti-cheat programs, maintains a list of games it is featured in. Comparing it with ProtonDB, the issue becomes clear. In addition to the games mentioned above, titles likeFall Guys: Ultimate Knockout,Outriders,andDead by Daylightwon’t work.

There is some good news, though.Valve saysit is working with Easy Anti-Cheat and BattlEye — another popular anti-cheat program — to bring support to the platform ahead of launch. The issue isn’t with the anti-cheat software itself, but rather that’s built to run on Windows, not Linux. If developers can include Linux support, we could see support improve.

The other option is toinstall Windows on the Steam Deck, which is possible. However, Valve’s own SteamOS build will likely provide the best experience, as Windows is built to run on full PCs that include more powerful hardware.

If you primarily play single-player games, you’re in luck. Games likeMonster Hunter: World, Cities: Skylines,andRed Dead Redemption 2run just fine with Proton. Many multiplayer games do, as well, includingThe Elder Scrolls Online, Among Us,andRocket League.