As much as I love James's delivery on the older albums, Charlie is a perfectly fine singer. He had quite a unique sound compared to other prog bands in that scene. I don't see his presence on WD&DU as a detriment to the album at all. 
I think Charlie did his best, and that was quite good tbh, I have no problem with his singing on WDADU, except maybe for THAT part on TKH...
Imo, he just gets the hate because James is so much better, that the contrast doesn't make Charlie look good.
Yeah, that's a good way of putting it. I think there are two things that kept Charlie from shining on the album:
(1) He wasn't really a
metal singer at that time; and
(2) The songs weren't written to play to his vocal strengths.
WDADRU is an interesting example, when he is singing next to James. James does fine, but it definitely wasn't one of his
best nights. And by the time they got to the encore when Charlie came out, James' voice was obviously tired. As a result, I think James sounds thin, and there are a few moments where he is a bit shrill. Then you have Charlie's voice right up against James'. And while Charlie clearly does not have the range and power that James has, his voice has a warmth and depth that James' doesn't. And contrasted with James' more tired delivery
at that point in the set, Charlie sounds better than he otherwise would have. That isn't meant to be a knock against
either singer. To me, James
clearly does a better job with all of those songs. But we got to hear Charlie years later, doing only two songs when his voice was fresh, and he sounded pretty good.
Then you take Charlie on his solo albums. He sounds really good on all of them, and there is a definite, marked improvement just over the course of those three albums from O1 to O3. He was finding his voice and finding a way to sing metal that worked well for his voice, and the result was pretty good. AND those songs and vocal melodies were written to showcase
his voice, and not with his voice simply being an afterthought.
To this day, I still don't like Charlie's voice on WDADU. It doesn't work for me. And the above are the reasons why. But until he resurfaced on WDADRU and with his solo work, I assumed that I just didn't like his voice at all. Hearing him with more experience in this genre and doing material that is better suited to him changed my mind.