I don't believe it's fair to make an average player like me try 10 times to win - it's not fun. I'd say fun is to give your best and to win on 1st of 2nd try
Master level and higher players probably constitute less than 5% of the CoOp player base, why would you make the mutations fun only for this group?
If someone wants an insanely hard mutation there's the custom option to add several hard mutators at once
I did all the campaigns on brutal - why should mutators be any different? Just challenging like the harder missions seems fair to me
I'd also say the hardest missions in the campaigns where you had to either abuse a specific mechanic, or be a pro, or save&load tens of times were not fun either