The design is okay. It doesn't do fancy tricks, and it keeps pretty much like the original way of designing. In this huge level there are many doors in the main level, each brings you to a room that will make you start over until you find the right door. Pretty silly idea, but it could work; of course, the fact that you have to redo the first sublevel each time you pick the wrong door is a bit frustrating. The rest is pretty much okay-ish, except for some small flaws like the one Counterfeit mentioned, or the sublevel where you have to blind fall randomly (which BG also is cutoff), hoping that nothing hurts you, or the one where the doors are hidden over a thwomp offscreen. It has its flaws, but it's not extremly bad overall.
Aesthetics are more or less good, except for some silly ways the author assembled some exgfx (like the SMB3 pillars BG), some palettes and some cutoff here and there.
As only major issue, what Cou said, sublevel 27 can make you stuck due to the way P-Switches are placed. Of course it's not impossible, but you have to take one hit on the spikes.
Scan Assessment: Room 27 is completely unbeatable due to P-switch placement. Will qualify for an easy "fixme" badge as no legacy hacks are to be removed. In addition, the music note bounce sprite should be remapped to avoid conflicts with the Grinder, so long as no other item in the rooms it's used in become corrupted.