My thoughts on a few issues:
Anders was never an exceptionally good vocalist. His early vocals, which are true growls, show absolutely no variance and seem too forced when compared to growlers like Mikael Stanne or Mikael Akerfeldt. Simple reason: Those Mikes' normal voices are fucking LOW already, and Anders' voice is high pitched and whiny. And I don't think he had a proper technique to do those, which might have helped in this situation.
Anders changed from growling to screaming in the span of two albums: Colony and Clayman. In Clayman and on, he purely screams (when he's not singing). In Colony, he had a unique style which looked like a combination of growling and screaming; which was I think the best vocal performance of him. Still not really exceptionally awesome, but I'd say very good. It would definitely be awesome if he stayed that way.
But he didn't and switched to completely screaming, which didn't work out too bad either, I think, because his already high-pitched voice is a lot more suitable for screaming than growling. And I don't think his screaming performance has changed much since then... if you omit the tons of vocal effects (I think his voice receives more distortion than the guitars...) put over it, more and more with each album. Still I wouldn't call it bad, it's "standard".
Clean vocals are a different matter entirely. In Clayman he used a lot of cleans, but they were almost always used in verses (he screamed at choruses) and it was more or less just whispering without any melody, which fit the album perfectly. But after Clayman he switched those places - usually he screams on verses and sings on the choruses (which is the PRIMARY reason why IF's music has changed this much: a melody that is to be performed by dual guitars is much more different than a melody that is meant to be sung; and IF had always been known for the dual guitar harmonies). Which sucks, because as I said, his voice is whiny and kinda ugly (sorry Andie); although I'll admit it's kind of unique and many people would like it. By liking it, I mean liking the "tone" of his voice. But about the other dimension of singing, which is "your ability to sing", Anders fails unquestionably. Why do you think IF is releasing bad album after slightly less bad album now? Because they are writing melodies for a style they aren't famous for: singing, and the person which they write those songs to sing, actually can't sing. I don't care about sales (which I'm sure are better than ever), but for me, IF cannot win musically with this strategy.
About live performances: First, I don't think that video was that horrendously bad, he seems to be getting better as the song progresses; and second, I think he's just really tired. It doesn't have to be anything about his throat, just look at the way he talks to the audience at the beginning, he's just VERY TIRED. I don't know if Anders got over that period and is lively and kicking right now; but if he did I don't think his performance would be worse than the last year or something.
But the fact remains that his live screams always sounded VERY forced. And I don't need to tell you about live cleans - even his album cleans suck.