Meliadoul wasn't a bad character per se, it was just Orlandu was too good of one. Had Orlandu not been the powerhouse he was, Meliadoul would have been a lot more useful to the party.

Orlandu outshadows Meliadoul because he has all her skills plus Gafgarions and Agrias's. Since the party is only so large, you select Orlandu over Meliadoul every time. If they had given her a better or different skill set, she'd be a beter asset too.

As far as Izlude is concerned, I see him as in the middle. He could have chosen to listen to Meliadoul but he insisted upon siding with Wiegraf and Vormav and ultimately he stayed with them too long.