Featured Game Modes need to use normal game MMR
Call me an asshole, but playing them is next to 0 fun if you only ever get to play with Bronze 1 through 5 people, with the occasional silver. I don't care if I have to wait more than 5 seconds currently to find a match if the game quality is going to be higher. Getting flamed for picking the undoubtedly strongest champ Yasuo in Onslaught difficulty (you block every huge damage attack and you can perma cc entire groups of enemies + long invulnerability and low cooldowns) and having people ragequit because they try to 1v5 mobs that hit them for 50% of their HP is impossible to deal with.
"It's not that serious it's just a fun mode", but if you want to beat higher difficulties you need some kind of idea of what you are doing. Being at the mercy of 4 other people is never fun.
"Just ask 4 of your friends to join" Yeah, I could do that. But it's still a shame that I HAVE to, just to escape from people who don't even bother equipping augments or bother reading what they do in the first place.