There are two Units I'd like to mention here, although there are probably many more Units I regularly use, that could be declard bad.
1) Pelleas. It takes way to much effort to get him, and then you'll have to invest realy much, for him to get somewhat mediocre. But still, everytime I get my hands on him, he will be a part of my team.
2) Hugh. He is realy realy expensive, for the little he brings to the table but I'd train him anytime.