A short hack, but not something I'd recommend. Music doesn't break either, as it's all the original tracks.
Chuck Island 1 is mostly an okay level, but I couldn't figure out what to with the red P-Switch, because I couldn't read the first time around. The message box tells you exactly what you need to know.
At the end of the cave portion, there are these air blocks just below the pipe that prevent you from entering if you hold an item, or just push you left, and can kill you if you get pushed into the solid wall. These are likely there to prevent you from carrying Buzzy Beetles over to the next room, where they would appear glitched.
Chuck Island 2's first half is simple, reasonable, well-balanced, but a little short.
The water portion is also of fair difficulty, but having the Rip Van Fish in such a small tunnel is not a good idea. Maybe the purpose however was to lead it out of the tunnel to bypass it.
Chuck Island 3 is the huge difficulty spike of the hack. You lose your powerups upon entering all of the enemy based segments, meaning one-hit deaths. On top of that, there's precise jumping across small platforms, bubbles, a few moving Koopas, and even a few blind falls. This is what really killed the experience.
Chuck Island 4 and the following castle return to a more reasonable difficulty level, as the powerups are back.
If anything, the huge difficulty spike is simply unreasonable and it screws up the overall hack. It was only a demo, sure, but holy Jupiter, that was such a huge spike.