On a fresh install, phpbb 2.0.22 is compatible with MySQL 5, but a lot of MODs out there are not. If the MOD says "MySQL only", it will probably NOT be compatible with MySQL 5.
Why would that be? Because MySQL 5 enforces rules that earlier versions did not. These are the same rules that other databases enforced, which is why a lot of MODs would not work with other databases... they didn't follow the rules.
The abuse will continue until morale improves!