I also think it's pretty safe to assume that a Thing Stolen combo for RW is pretty far fetched, since it's usually not even worth it for a normal Mordor squat deck and you need two RW present...
The critics focus more on the 'filling the holes' aspect of getting MP's, than on the total mp advantage I suppose. It is very useful if your main company doesn't have to think about getting an ally when burning free-holds for example, because at some point your RW can get it. Also, but to lesser extend, for heavy faction deck, getting a single item MP for Blasting/Liquid fire might be usefull too. Of course it's easier to tap a RW at a shadowhold, but you have to play only 1 ally, or 1 faction, or 1 item, during your 5 turns, so opponent can't keep tapping him each turn probably.
Getting multiple Fires means designing deck in a way that cardmanagament deals with less slots for support cards, and also you'd need Uvatha to join your RW to really take advantage. Then there is the disadvantage of not having sideboard or Voice options for your RW if you need him to play mp's. So this works only in limited cases.
With regards to factions, since your RW can play one at darkhaven already, the question is more: how big of a risk is it to have only 1 faction to play, and consequently how big of an advantage is it to have the option to play several with your RW? (e.g. faction + snaga, or at least have back-up faction in deck should you fail the roll)
Notwithstanding, I don't think this UEP is overpowered either. Here my breakdown:
Ren can get the obvious benefit from his home-site, but his ability is not that much used, and anyway if you tap him alot for his ability (for ring decks) you can't play anything unless you spend cards to untap him. Maybe we can devise a restriction for not letting him tap during the Audience unless at darkhaven.
Dwar has to tap also for ability, so same story, and he can't play ally at home site. If it's for just the faction he can stay at haven also.
Uvatha has to tap also for ability, faction is worth only 1 mp more. But, with a Black Horse V he does have a good ally advantage
Indur, has to tap also, but does have a useful ally site, albeit very dangerous so he needs cards to support at least (which he can get himself by tapping).
Akhorahil, ability is only really useful if he moves I suppose. Though the magics can help him score good faction points at Nurniag Camp (5), I doubt it would weigh against the character disadvantage. Of course he has the ally advantage also.
Khamul, home site offers no advantage.
Adunaphel: ability needs tapping, and home site offers a real advantage only if she taps. No allies or factions.
Witch-king: useful since he can get Indur and Uvatha to join him at Shelob's Lair/Cirith Ungol, thus making it possible to actually play usefull things, or alot of things...but, he doesn't need this UEP to do that!
That leaves us with:
Hoarmurath: no allies playable, possible 1 extra faction mp but very hard roll, and of course he doesn't need to tap for ability, so he could focus on playing alot of Fires/Fumes, that's not nice...
did I miss anything? then lets continue with the thorough analysis
