With any sort of code, there is always the potential to crash or glitch the game, then lose any progress you have made. Sometimes, this can add to the fun, though.
The code that allows Player 1 to be the CPU is interesting, as it pitches both fighters against each other. This does not always seem to work on Arcade mode. It works in Endurance for one round, after which it freezes the game and you have to reboot the PSP. It works best in Practice mode. For regular fighters, there seems to be a lot of shuffling of feet and not a lot of actual fighting, unless you allow the game to go into Demo mode, where the fighters will use their special moves and combos. However, if you set Onaga against himself in Practice mode, the whole dynamic seems to change, with both character using special moves a lot more. You will still have to reset the game to change characters or do anything else at all.
If you deactivate the Player 1 CPU code and just use Onaga as a playable character, you can use special moves (see the section on playable kombatants) and arena weapons. It is very difficult to use arena traps against Onaga, which is obviously in your favour if you are playing as him. What happens is that Onaga tends to be knocked out of the arena and just stands in mid-air, rather than being affected by the trap. Or the yellow or red line acts as a wall, beyond which Onaga cannot pass. This renders most of the death traps useless against him. On any multi-level stage, this glitch can then affect the other character as well, with both fighters standing in mid-air.
However, it is possible to get Onaga on the Falling Cliffs stage, if both characters are caught in the collapsing cliff: a different take on the suicide moves in the game if you are playing against him.
In general, though, the players move around the arena as if the different levels do not exist. With Onaga’s air throw, this leads to some rather odd camera angles. If there is a ceiling on a particular stage, he may disappear right through it, or the camera may zoom out to an extreme distance as he performs the air throw in an open air stage. In other places, there may not be any fighters in shot at all, allowing an unrestricted view of the different stages. Of course, you can push the fighters right off the edge of the stage and into a grey void, which is very easy in the alternative Nexus stages.
UPDATE: The Player 1 as CPU code can be used in Kombat Chess. The computer will play itself and fight a lot more vigorously than in Endurance or Practice modes.