Derek is right in one sense, that a lot of voices in the genre are subpar, or were once good but have not held up.
That's still not an excuse for an album's worth of afterthought vocal melodies.
That's kinda the trade off though right? Its something that has been discussed here a lot regarding DT. If you write vocal melodies in the studio that are easier for a singer to pull off live, you are basically limiting the singers range (in most cases). This often can lead to monotone vocals that don't have a lot of movement, aren't terribly dynamic, and tend to be forgettable.
On the flip side, you can do essentially what DT has done (for the most part) and continue to write pretty challenging parts in the studio for James. He tends to pretty much always deliver in the studio, which leads to a dynamic performance with lots of range and movement on the record, but he tends to then struggle with those tough sections live.
I'm not saying one way is right and another is wrong, just different approaches, that's all.