With the exception of the Mouth of Sauron, these are fairly high cost heroes, and it can be difficult to realize their full value unless you're playing fairly high points level games (say 700 and higher). Simply because to play one of those heroes reduces your overall troop count so much that it's too challenging to provide adequate support for the big hero to be able to do what they do well.
At 500 points, it's much more common to see lots of "mid-level" heroes in the 50 - 90 point range, as they can provide value, but still allow for enough battlefield core troops.
That being said, a few specific thoughts:
Re: Necromancer: I get your point about 2 wraiths providing more value. Can't really argue too much. The Necromancer can be a brutal hero killer if used well. But can he "earn his points?" Hard to say. I'm not sure if the old Mordor Sourcebook profile is now defunct. I thought that that wording only applied to previous "Hobbit" profiles (An Unexpected Journey, Desolation of Smaug, Battle of Five Armies), but I could be wrong.
Re: Gothmog. I actually quite like his special rule. And I don't think he's overcosted. His cost is pretty similar to your standard high-ish level hero with 3 Att and 3 M/W/F. Close to Glorfindel or Eomer Knight of the Pelennor. And I think his special rule can be super handy (and extremely annoying).
Re: Mouth of Sauron. You're in consensus with most of the community on this one. He's OK at combat, but only has 1 Might. He's OK at spells, but not quite as good as a wraith. There are others that do all that he can do, and do those things better.
Re: Gulhavar. I think he's pretty damn scary. 12" move, fly, and a combat beast (F6, Str 8, 3 Might), in addition to being a monster, and gaining access to Brutal Power Attacks. Monsters with Might are *really* scary. He's both an insanely good hero killer, and also capable of just smashing up an opponent's shield line.
At only Def 5, and because he has Terror, yes - he's going to be an archery target. Keep him behind friends until he can engage. Legolas though does present problems for him, and at 200 points, he will be a Legolas target. But Legolas should only wound him 1/3 of the time, unless he uses Might. And if Gulhavar stays with your army, and can wrack up some kills, he should be able to keep regaining wounds. Ideally, the rest of your army can tie up the opponent enough for Gulhavar (or something else) to single out Legolas and knock him out. Be careful not to expose Gulhavar away from the rest of your force though. He will go down fast if trapped.
But your opponent shouldn't always have Legolas. And hopefully, the rest of your army can try to get to Legolas, or at least
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