The problem with putting stuck head cement bottles in microwaves is that some head cement bottles come with metal caps.
For example, Wapsi Gloss Coat and some bottles of Umpqua Head Cement.
Metal should not go into a microwave because it can cause arcing. The results are unpredicatible.
Microwave and metal
Secondly, because nothing bad has happened does not mean nothing bad will ever happen. Putting a sealed container of volatile material that can expand and pressurize the sealed container beyond it's burst rating is risky. Maybe it will and maybe it won't but if other methods are safer, why do it? Hot water on the cap is safer.
Heating uncovered materials that cannot pressurize is like comparing lighting gunpowder in an open flame vs a pipe bomb.