The m10 lower penetration than stug but has AP rounds and speed boost to flank.
At hefty muni cost in most muni heavy faction in game.
It has a different play style than Stug.
In theory.
In practice, unless it would be desperate dive against KT or ele, it would be used exactly like stug - from max range in the back line, because there is no reason to use it in any other way except the two aforementioned units.
And trying to balance units vs enemy and allied units should not even be attempted. In team-games each faction should be using the stronger tools in its arsenal and not try to provide all types of units.
Yeah, no, that isn't going to happen as long as armor homogenization and generalization(which you are in full support of, if you have forgotten) continues.
Also, there still is issue of not being able to reliably counter heavies or panthers if M10 thing went through(without further changes, as Sander explained).