I've been exploring FUSE a lot as a source for characters for a UE4 game I'm trying to develop. I had a month of the "indie pack" (not on steam, but through their subscription) and successfully got a FUSE character moving around in the UE4 environment.
FUSE is unique in that it's essentially middleware that's trying to do the jobs of 5 people in a team that usually consists of 10, so it won't be able to do everything, and doesn't claim to. It'll be able to make awesome looking characters that are rigged for very cheap, and on their site, there are animations you can buy. Or, because the characters are rigged, you can animate them on their own.
Will FUSE characters be able to immediately drop into (name of program)? Probably not. They might need some coaxing. You might have to get really into (said program) and learn what it is that makes it work... but this is a good thing.
For UE4, a hang up was on the skeleton and the material quality. For months, I was going on their forums and freaking out about how a UE4 skeleton download option would be great, or how sometimes the materials didn't come through looking the best - because UE4 didn't naturally know how to handle some of the FUSE character's files. Well, after months of awesome awesome customer support on thier forums - they came through with that skeleton. Plus, not that it's their job to fix things in a totally different program for me, but they're planning an update for the UE4 materials.
I'm a believer. These guys are keeping pace with the bigger game engines and are inching closer and closer to that "dream product" you want it to be. They've bitten off a lot, but they're delivering, and i'm very excited to see where this goes - and to use it in my game.This is a tool, and an awesome one, but just don't expect it to go ahead and make your game for you! That's your job!