how reliable is pg_rewind?

When trying to resync an old primary to become a new standby, I have found that pg_rewind only works occasionally. How reliable/robust is pg_rewind, and what are its limitations? We have observed that approx half our FPIs in the WALs are due to XLOG/FPI_FOR_HINT. The only reason we've set wal_log_hints=on is so that we can  use pg_rewind. But if pg_rewind is unreliable, we would rather turn off wal_log_hints. Any info on the reliability of pg_rewind and its limitations would be appreciated.