I agree with you re the knight- it was created to tick the every army big monster tick box that is next to the flyer tick box, that said I still think its pretty cool.
For the jet bikes I think the main problem is the torsos of the riders, If they are swapped out for guardian ones they look much better. Shame this is a really tough conversion
Also agree on the Phoenix lords (I'd throw in Eldrad into this if he's still in the book, as he's technically dead), they look pretty awful, and very flat. Jain Zar is probably the worse and Karandras is the best but still