Some PSX romhacks break the ECC Data. This results in those romhacks not working on both real hardware and accurate emulators. Typically they're only testing the romhacks in something like epsxe, which doesn't care abou ECC Data.
All hope is not lost however. To fix this, users can use tools like ECCregen to repair the game after patching to make the games work. However, this is an extra step that adds complication to the patching process, and is completely uneeded if romhackers had just patched their games properly in the first place.
So, get the word out that PSX romhacks should patch the ECC data as well, and to test the romhacks either on real hardware (ideally) or high accurate emulators.
Also, romhacks that make use of ASM hacks, do not work on real hardware under any circumstances as far as I know.