I'm a RM adept
I just think they're cooler, and more fun to play.
In SP they add to the challenge too, in MP they must be protected, but offer interesting diplomacy options, 'cause you'll have a use for any type of gems or item building ability : trade !
And you aren't stuck with low path level neither : don't empower (too costly), but build magic enhancing items of all kinds ! Those are generally available at Cons4 with path3 or Cons6 with path1. In the end your RB can have whenever he needs it level 5 in any path, cast potent spells or build powerful items.
OTOH I've been disappointed with "SC Pretenders" : bad research at start, not generally needed for early expansion, and too often getting afflictions or even get killed in battles ... High level magic don't give a big advantage until you have a good research.
Plus you have to invest heavily on them (items), it's too much in the same basket. Path9 blessings need heavy design investment, and are good for nations with many good sacred units, but ain't that much powerful with other nations.
Immortals are cool however, and some creatures such as Ghost King are a good compromise.