The downside is that for the most part, Samus is really the only character that is easily playable, which is surprising since she's perceived as a silent protagonist (in spite of her speaking roles). Beyond them, only Adam and Anthony have any character development to speak of. The Prime 3 Hunters have a little work with, but not a lot and we know so little of their background. The Hunters from MPH have no speaking roles at all, so any personality for them is entirely guesswork.

So when it comes down to it, you have a playable cast of Samus, Adam, Anthony, Ridley, U-Mos, Dane, Gandrayda, Rundas, and Ghor. Not a whole lot to work with, unless you start drawing from the manga too. Which is too bad, because I really love the Metroid universe (can you tell?) and love to see it explored more.