Does Baleful Polymorph work vs
- constructs?
- corporeal undead?
- incorporeal undead?
None of the above seem to have relevant immunities that say otherwise, so yes. The notable exception being golems who are immune to most magic.
In general, PF1 cut way back on monster immunities and resistances to limit them to things that specifically make sense for the monster, like a fire elemental being immune to fire, rather than a vampire being immune to mental effects. In exchange, the incapacitation trait and save or lose effects being limited to critical failures means you're less likely to one shot a boss monster with things like Baleful Polymorph. But your spells still do something on a successful save. The net result is that spell casters will almost always have something to do rather than be shut down against certain foes, but won't be soloing encounters as much.