Different classes have different roles as it is now, the mut just make things unbalanced toward above-said roles.
The soldier is an all around class that has no "natural enemies" but I agree about the demo- they are designed mainly to counter engies. But what can you do when you encounter many other types of enemies on your way?
I dont think they are overpowered- good scouts, pyros and soldiers -not to mention spies and snipers- can take a demo out. Dont forget he's the only class that has no gun and his reloadings are not the fastest.
I do agree that there are certain locations where a (good) demo can camp and its close to impossible to dig him out.
I do know where your coming from , and the soldier is the most all-round class but i still think of him (or maybe like to think he should be) countering static/slowmoving targets, engies + their sentrys etc. as well as forcing snipers and others into cover.
As for demos- who can kill by landing nades on the volley, in the confined passages typical of most maps and with 4 fast firing shots a skilled one can floor a small group easily. And besides, really in an idea situation, he should be escorted forward by other classes.
Having seen skilled demos (and to a lesser extent soldiers) completely mop up anything they face with no need for support from other classes, I thought phaps reducing their effectivness against inf by removing crits might be a good idea.
Then again- you've played this mutator not me so who am I to comment on it's effect, positive or otherwise.