I just hope GOG techies made sure unpatching did not reintroduce the random number/ring of greed bug of the original digital download version that forced GOG to use the unofficial patch in the first place.
If constantly failing to cast spells and having towers always shoot at the same tile over and over again is due to the random number generator bug (I'm guessing it is), then the new release has it.
Althought I remember the bug more from producing Ring of Elven Greed as only loot, your description seems to confirm the bug is back unfortunately. Bug description from way back when:
"The problem is that all random events in the game like spell chance critical hits...etc is broken. Nothing will happen unless it's 100% sure to happen. This makes most spells useless as well as all random items since all you get is a Ring of Elven Greed no matter how many chests you open."
Supposibly Version 1.03fix
(alt download location
) provided by Ahatch fixes the issue and removes the controversial changes made by unofficial patch.