I have a (twitter) friend who went on sub (with a great agent) with a YA super hero romance (think sky-high as apposed to superman) and publishers had nice things to say but wouldn't bite. She's now self-publishing and I have no doubt she will do fantastic (she doing an incredible job of promoting it. Has a amazing professional cover, copy-edited etc. The buzz around the book is huge (for a self-pubbed book) and it's not out until next month)
Of course this is YA. MG is a different beast. I think there is a market for the right stories, but publishers seem hesitant to take a risk on it. This doesn't mean you shouldn't write it, or sub it, but it is nice to know if it's going to be an uphill battle. Which it might be. But again, I don't know how this relates to MG.