I'll put a video in a little bit the only scenario I could see it failing is if someone holds down and does a delayed wakeup but at that point the charged palm would be over and you'd be able to block so it may only be a fool me once kinda situation :/. Marvaz's setup you can jump out of but I'm not so sure about the 21F2 setup either way it is worth trying out xD.
I dont think you get it, cage has no untechable set ups, which means that on wakeup they can do a wakeup move, block or jump, and if they jump even if the palm hits it wont combo so basically its just wasting damage or being greedy
On top of that, doing the palm while theyre knocked down gives them more options than you, you can either cancel into block or a normal move. The normal move will be beaten by the opponents wakeup because they will have invinsibility frames, and blocking has just switched the pressure game to their favour. This is why the palm is better to do after a nutpunch because they have 2 options: Move or block.
The opponent can do way more, they can cross you over for one. If you dedicate yourself to the palm you'll just eat a full combo.
They can wakeup, which will beat ur palm anyway.
They can roll back, meaning that you just lost a load of damage and now have to clear space again which is very difficult at high lvl.
And like you said, they can just lay down.