Attitude towards complete anti-magic!

The Antimagic spell is still useful. Personally, I'm willing to sacrifice the ability to cast any other spells on a hero or unit.

but casting positive magic grants 100% immunity... in my opinion, that's a cheat...

I agree. If it's immunity, then it should apply to positive spells as well, like with the Necromancer.

and still, Antimagic as an ability is a gameplay imbalance... IMHO

There are many such imbalances. For example, necromancy. And it really does unbalance the game a bit.