Playing an ice elementalist to D:4, so far I have twice been almost killed by miscast effects.
Playing the way I'm used to (casting Ice Beast at 36% fail rate to prepare for a difficult encounter) apparently isn't a good idea with the new settings
Of course the severity of miscasts can be discussed separately from the kinds of miscasts, thanks to your simplifications
I don't really like the severity of the miscast effects. Maybe it's just because I'm used to it, but I like getting a power spike when Summon Ice Beast is quasi-castable. Without them I'll have to run away from stuff more and I'll be significantly weaker than a melee dude at the same XL.
It does also sometimes happen that a spell fails at
any non-0 fail rate, and at this XL the loss of 4mp and a turn is often enough to make a situation risky. With the severe miscast effects I'll have to take more precautions than I'm used to, which makes the game boring-er. I anticipate that perhaps I can't react to a dangerous situation by summoning a few ice beasts because if any of those spells fail I'll be dead (or need a blink scroll).
WRT the kinds of miscast effects, I'm a little concerned that ice damage might hurt a lot more in the early game than the late game (when you can get rC+++)
and nameless horrors are really difficult to take out for some characters, whilst being relatively easy to take out for other characters. I'm talking here about characters who otherwise have little trouble navigating the dungeon. If they stay this way I'm going to have to skill some characters specifically so that they can take out a nameless horror if it shows up. Currently it looks like you need ranged blast damage or non-magic melee damage to be able to deal with them. Some build options are lost, which is a shame.
If you'd make the nameless horrors go/melt away after a while, at least getting hit by a summoning miscast wouldn't come at a large strategic cost to certain characters, and I think they could still be as dangerous as the direct-damage miscasts. I understand you can't make them summons because they'd be susceptible to AoA, but StS shows there are also other options.