Well first of all we have amazing miscast protection. Cupped Hands, Soul of Stone and Lore of Life makes miscasts a joke. If you get super unlucky and still manage to miscast, then yeah its pretty bad if you play with TGs and pretty harmless if you go solo slann.
If you play with TGs then its often game over for your TG unit (unless you play a death star but either way its gonna cause huge amounts of damage): Cascade kills 90% of the TG unit and has a 50% chance of gibbing the slann, Calamitous Detonation kills like 7-8 TGs, Detonation kills 7-10 TGs (depends a lot on unit size). That means a roll of 2-7 kills a huge part of your TG unit.
For a solo Slann Cascade has a 50% chance of instagibbing your Slann but if you roll 4+ then its not bad at all. The detonations and feedback are harmless, they might cause an odd wound and ruin the rest of your magic phase but nothing game changing. Power drain is bad, but at least you don't give up any victory points. Basically you need to roll 2-4 and then 1-3 for a miscast to be a catastrophe (loosing 3 magic levels isn't fun either tho).