Consider this. How do you knock a gelatinous cube prone? Is it like Bob the tomato where the difference between standing and sitting is a quarter inch crouch? I understand why oozes can’t automatically be immune to being prone. Lest we forget being a thief in third edition D and D could be a real hassle. If your DM liked undead (or golems or oozes) you pretty much couldn’t do your thing. So all in all I like the change to a more equitable game. As a player I feel useful more often and as a DM I can try to overwhelm my players with my undead hordes without worrying about invalidating someone’s character. But, somehow an ooze getting knocked prone still doesn’t make sense and I’ve wanted to use the concept since the start of fourth edition.