Why should I be able to use a spell to push Cazador in his gaseous form down a chasm. He could easily just glide back. You have a system like that already implemented for the mindflayers in act III right before the final battle (unless this was a bug). I would use it for every character that is capable of flying, teleporting or has a similar skill. Give them one turn in mid air where they can use a bonus action to come back.
Yea I feel that if a creature is capable of flying and isn't stunned, they should be able to fly back up from being pushed off a cliff, otherwise it makes no sense.
Not to mention that under the Gaseous Form spell, the description literally says that [a creature in gas form] can't fall.
I also recall fighting a bunch of humanoid fish people in the Underdark and when I pushed one into the water, he just came right back up. I chalked it up to the fact that since the enemy was a fish it could swim, therefore it made sense and was most likely intended.
Speaking of gas form, does it make much sense that while in gas form, you can still be physically restrained by some attacks? I'm not familiar with DnD so maybe it's an expected mechanics and i just don't know about it, but it did come to me as a surprise that during the final boss battle, the
were able to grab and restrain my characters in gas form. It felt counterintuitive to me but if that's the intended interaction then i have no issue with it.