I will say that the immortal tower super life strategy doesn't really work out too well on hard and very hard (especially very hard). It becomes a guessing game and your lives bounce around so much, that sometimes you lose and sometimes you stay alive during certain rounds (namely fires, and undead rounds). Having played lots of combinations, immortal tower strat may be just a bit better than average, but for the most part, it feels about the same as most others.
As for Comet/hail. Yeah, these are slightly overtuned. And it's not so much the tower itself as it is it's combination with trickery lvl3. Getting an 80 second tower for the low low cost of 1900, especially with hails (3500), effectively doubles your towers. And with the cooldown on trickery's being low enough that with 4 towers, you can guarantee 4 doubles up at a time... well, you essentially get 9 towers (this includes the trickery itself) for the cost of 4+trickery. I'm not sure how to fix this though. Perhaps uncoupling the dark requirement from hails and adding a different element if that's at all possible. Or straight nerfing the trickery tower perhaps. Hard to say. All I can say is that with comet/hail + trickery I can easily get to fruit lvl and maybe lose 1 life on hard without trying (the strat easily deals with earth rounds) and still get 300+ fruit points while maximizing the interest savings.
I don't feel that it's the range or damage of the hail tower at all either, rather it may just be the number of additions it gets from it's buffs. instead of 5 perhaps 4 is the simple balance it needs. I think a couple different solutions could be tested.