Also, while the mana impact on rares+ is rather insignificant, the current value of Mana Clash is wiping vim/pos/neg.
That's the problem. The ability is called Mana Clash, so it is weird that it is good for many things, but not for anything to do with mana..
I like the simple solution best: give bosses / rares a much more realistic (smaller) mana reserve, so that Mana Clash can actually drain it. And then give them a mana surge rune, to give them a way to recover.
Regarding fungus: honestly, I dislike Fungal Growth. By itself, it has a purely negative effect, because it will trigger a weak regeneration effect when you heal, which you have to remove, because it blocks your real regeneration (infusion).
In combination with Ancestral Life, it has an effect that is both annoying and overpowered: free turns (OP), but you still have to cancel the weak regeneration every time (annoying). Add Fungal blood and it becomes even more annoying and even more overpowered.
And worse yet, it plainly makes no sense. Free turns because you trigger some minor regeneration effect??
So could we please put an end to that? If we replace Fungal Growth with another ability (boost to healmod? improvement to heal infusions?), it will make fungus more balanced, and easier to use at the same time.