The difficulty scaling in the game is rather uneven. For example misssions of levels plus:
+8 is -84% damage dealt, +264% damage suffered - i.e. you are dealing about 12% of normal damage and taking 2.6 times damage
+9 is -90% damage dealt, +334% damage suffered i.e. you are dealing 10% of normal damage and taking 3.3 time normal damage
+10 is -95% damage dealt. +412% damage suffered - i.e. you are dealing 5% of normal damage and taking over 4 times normal damage
+11 is -99% damage dealt, +499% damage suffered i.e. you are dealing 1% of normal damage and are taking 5 times normal damage
+12 is -99% damage dealt, +594% damage suffered i.e. you are dealing 1% of normal damage and are taking 6 times normal damage
+13 is -99% damage dealt, +697% damage suffered i.e. you are dealing % of normal damage and are taking 7 times normal damage
Theoretically speaking the progression from +8-+9 is about 35% harder, from +9 to +10 is about 65% harder, from +10 to +11 is about 600% harder and from +11 to +12 is about 12% harder, +12 to +13 is about 17% harder.
In short, the jump from +10 to +11 is massive and the jump for every level after is tiny.
I think the difficulty scaling could do with some revision especially since every build that has been easily capable of of +11 and beyond has been on the receiving end of a heavy nerf, such as vulnerability multiplying with the Uncreator rune and Berserker builds.
I would suggest that instead of jumping from 95% damage dealt debuff to 99% damage dealt debuff, values between 95 and 99 are considered since every 1% change increase from 95% is effectively huge.
I would also state that effective builds at the 99% damage debuff (i.e. levels _11 and beyond) are few and far between and force players into using very specific "meta" builds to be able to even complete missions of this difficulty.