I have no problems with autosaves per se. They're not fireproof. "oh, do you have a save at 1? well enjoy this dwarf cannon buffed by 3 runelord. 2d6 dam at rend -5".
What I don't like is:
1- to stack bonus on bonus, the battle will become a game of "Unstoppable force Vs Unpassable defence". It drains my mental resources from the fun part of it.
2- It will favor lists heavy oriented toward Mortal Wounds. They will grind casual players in zero time.
3- If you cannot have MW and you're facing a Bastiladon, you lose. "Seraphon are overpowered, yadda yadda".
4- Autosaves are relatively easy to obtain. Lots of mortal wounds, or really high rend attacks, are less immediate... we'll be back to copy&paste lists that hit hard, thus losing the "play what you want, it will work" part, that was one of the main points of AoS.
5- If your opponent is not good as you, he won't have a chance... what's so wrong in "1 is always a failure"? I don't know you, but I'm not having fun if I'm 100% certain of the outcome.
I'm sure there's something else, but i think you get my reasoning.