My concern isn't that I don't trust Keybase to not be malicious, it's that I don't trust their packaging to not conflict with other packages. Debian has a very strict packaging process and it effectively guarantees a stable system, but installing packages that don't follow the standards that their packagers have could cause problems on upgrades.
I don't know enough about Linux to verify that the Keybase package does everything right; I delegate that to the Debian packagers and don't install anything as root unless it's from the Debian package repositories. Any software that I need that isn't in the distro is installed to a folder inside my home folder, where it might conflict with other custom installed software, but at least it won't break the entire system.