Less important now, but they're unsafe to use prior to PG 10.

As of 14.1, they still don't support unique constraints:

   select amname, pg_indexam_has_property(oid, 'can_unique') from pg_am

   amname | pg_indexam_has_property 
    --------+-------------------------
     heap   | ¤
     btree  | t
     hash   | f
     gist   | f
     gin    | f
     spgist | f
     brin   | f