Polymorph overall in 5e is lame.
You are limited to beasts only. there are no better versions of polymorph on higher level, up until lvl-9 Shapechange. Which is entirely opposite to polymorph - entirely OP.
And BG3 did it even worse. Generally, it's not worth polymorphing other classes, since they cannot use their equipment while polymorphed. Ranger, Druid, Paladin... all those classes will perform much worse, unless for some bizarre reason they still had only basic gear on level-7+.
But it's still worth using polymorph on familiars/animal companions. Polymorphing those to T-Rex.
Except... in BG3 you can't. They brought only most lame version of polymorph - baleful polymorph, and not the one that let's you polymorph allies into something "better".