Grease being flammable is a very popular homebrew, considering that grease is flammable (often enough, anyways, that 'non-flammable' grease is the exception). Additionally, PC-rangers cannot summon their minions mid-combat. Other than that, I agree with most of your critiques/questions.
I even have it in my own homebrew honestly, my alchemist Wizard PC wants all utility spells but pushes the rules to extremes to try and break the action economy for spells. (I let them DROWN a Gnoll with the Grease spell because they claimed the target was inside of them for instance, I'm against being too rigid BUT..) That being said, it doesn't work as well with a dispassionate by very nature DM ghost in the machine running the numbers in Baldur's Gate III since the enemies are typically smart enough to ALSO utilize grease in this way and there IS no mercy or alternative interpretation from the game DM.
I only think of it as a buff AND nerf because of the fact that enemies know it explode and will do it as often as they feel they can get away with it essentially wasting the Wizard's spell slot if you wanted to force the dexterity saving throws and difficult terrain. A majority of later enemies in D&D tend to have fire resistance or immunity built into them, which would make this a devastating mid to late game upset if we see any of those types.
