Probably giving nether no max level restriction is a good thing. But it has some minor drawbacks, like if you want to hold your teams level at least close to nethers’ level there is only one way now - leave it in stable for a bit.
Since when you breed your max or non-max level creatures with creatures from stables for greater heredity, stable creatures are obviously non-max level but considerably higher heredity.
Anyway, what i suggest:
give us some sort of item? (pill? some other stuff?) that once used, can halt nether’s exp/level progress, used again, can resume it - something like that. Idk maybe can be crafted from spare stuff like 1 book of wisdom + 1 upgrade pill. Since will be used more or less rarely, when your nether hits some level where you want rest of your team to be.
Otherwise it will always be considerably higher then the rest of your team. Maybe not a bad thing though.