The Tavern

Fear checks MR. If a monster can fail an MR check, fear should affect it. You can confuse and frenzy any mindless creature that can fail an MR check, so why not fear?

I don't think a whole subset of monster should have a random immunity to fear for no good reason. I don't think it's consistent with gameplay nor is it intuitive since you can affect undead with other mind-altering affects and you can affect living, mindless creatures with fear.

Alternative suggestion - remove fear as long as there is no way to know in advance what are the odds of success.
Some players like me don't even have an idea if it depends on MR or HD.
Current situation is weird, you can be lucky to affect Rupert 3 times in 3 different games despite success chance is something like 1% (I assume) and next time you will die reading the scroll as your last action(s).

Everything with success chances needs to be massively more apparent upfront. Starting with which spells SUFFER success chances so you don't memorize them by accident.