Balance shouldn't be too hard I think. Because most MOBAs have around 100+ heroes, while MH has around 25-30
There's a key difference though. MOBAs are built around the concepts of starting fresh each match and everyone having access to the same items within the match. I think it'll be easier to test for balancing in a situation like that. In Fire and Ice, you are bringing in characters that are of varying levels and different gear (artifacts, insignias, uniques, legendary, costume cores). Not only do they have to balance the heroes themselves, they have to balance the level brackets and each item (and potentially the OP item combos for certain heroes).
In the end, though, it's not a big deal since this is a just-for-fun mode. They don't have to be that serious about the balancing because it's not as complex (one lane, buying simple buffs, short match, not as competitive). I'm just worried they're putting too much on their plate with adding pve content, new pvp modes, new heroes and items, etc.