Alright there is one huge problem with this tier list that I see, and that is Ilyana's placement. Instead of being in Lower Mid, she should be just above Reyson. This is a very fair placement for her, and I will explain why momentarily.
First of all, when she joins she is superior to pretty much all your other units. Seriously. She has more MT than any of your other units except Nolan and Sothe, but she targets Res. Sure that is slightly counteracted by lolLow MT of Thunder tomes, but hey. She'll also have one of the higher Speed scores of any of your characters at the time, giving her one of the best offenses. Then consider she has alright durability and always attacks from 1~2 range, which means she pretty much can never die. Factor in the possibility of her getting transfer bonuses, and she steamrolls here.
Now, considering she has one of the highest levels of any of your units, she is probably one of, if not the first to get promoted. There's only really Nolan and Jill that will compete with her promotion speed. Once promoted, she gets better on pretty much all fronts. Really, her only weakness is her poor caps. As you can see, she courts them practically the whole game.
Also consider that she is necessary for funneling items over to the Greil Mercenaries! Without her, you've got no transfered items. Sure you might say that this doesn't require her to be used, but all that means is that she is a significant boon to the team even when she isn't trained at all. Imagine how good she would be trained. If we extrapolate from this, she is easily the best unit in the game.
Let's compare her to Reyson towards the end of the game. He can't even attack! She does way more damage than he does, and she's more durable! She can even heal with staves! There is no way that Reyson could be superior to her, except in mobility. Reyson relies on other people being good to get his Strength, Ilyana MAKES other people good by ferrying items to Greil mercenaries, AND she is an incredible unit by herself.
Again, this is a respectable tier list, but Ilyana is clearly undervalued. Just kidding.