You can use this tool to convert the deck.

The only downside I know of is that if your SteamOS install becomes corrupted (though a bug or from you messing something up), the Steam OS recovery drives won’t be able to fix it without wiping all your internal data. In comparison, with the original ext4 there’s a recovery option to only repair steamOS and leave your data intact.

This isn’t as bad as it sounds, you can still manually backup any of your important data before wiping and restoring the drive (you can access the BTRFS system from the recovery drive, and copy anything important onto a microSD or other storage device), but it’s definitely less convenient.