There's other issues. Like the fact that you could run out of memory and allocation fails. Then what do you do? For hard real time systems crashing isn't accept

There's other issues. Like the fact that you could run out of memory and allocation fails. Then what do you do? For hard real time systems crashing isn't acceptable, so you have to continue on, but assuming you actually needed that data you tried to allocate that's not going to be easy. So you need to prove your system won't ever allocate too much memory. That's pretty hard to do, and in most cases if you can prove that you can also write your system to use only static allocations.