The biggest problem with raid5 is that it is completely unprotected against silent corruption -- because there is no way for raid to know which data is the corrupted one (and as a result it has to decide whether the parity is correct or not -- though on most raid implementations just ignore silent corruption completely and so the parity is always assumed to be wrong in such cases).

So even if you rebuild an array, a bad drive might've blown away all of your data already. If you were to compare this with ZFS' "raid" Z1 (same parity, different design) you get detection and protection against silent data corruption.