Create a flat testing world and a platform at y 200. Stand on the platform and do /kill to clear all mobs. A few seconds later do / kill again and see that hostile mobs have spawned despite there being no valid spawning spaces within 128 blocks.

https://bugs.mojang.com/browse/MCPE-28352 says this is resolved and working as intended. If that’s true then hostile mob farms are a thing of the past without hours and hours of finding and spawn-proofing all the caves. And even then, spawn rates seems very very low compared to Iava edition.