Did some testing of the charged golem heart vs. The Bum's stun attack. The Summon Bats didn't stun 13 of 24 tries. Looks like about 50%. We'll have to see what higher levels do once those are available.
Important note for spaders: for the Summon Bats technique, you normally only receive the "You're totally surrounded by shrieking bats" the following pass if you opposed the Summon Bats. It does not not matter if you oppose the pass after that (ie. the technique that would have been removed).