I main Fuse but play Mirage in Arenas (one map) and love them both. Fuse is harder to play, though. He's brilliant at disrupting third party fights, getting enemies out of hiding spots and protection, but in a 1v1 or 3v3 as others have said it's just him and his gun(s). The KC is useless in a close fight as the animation takes too long. I get most of my wins/kills when I am carried (slightly) by a better or equivalent player, but in a squad where I am clearly the best player the games do not last long.
Mirage is less hard to play but you need to learn how to play him. Don't just randomly fire his clones off anywhere - send them off on a long, clear path where they are going to get noticed and not stop as soon as they hit an obstacle. If that happens you are basically telling enemies there's a proper Mirage nearby.
You can't beat the Life of the Party ult for giggles though. If being chased I round a corner, fire off the ult and then stand there waiting for the enemy. So many times they fire completely at a random clone and I down them with ease. That's a good tip - if you fire the ult don't start firing back immediately unless the enemy is spam shooting. Wait for that moment of confusion and panic when they turn so they are not facing you, then get shooting.