Back when i hacked metroid to learn the ropes (2013-14) of level design in particular, i was thrilled to discover that it actually works on an 8x8 pixel fidelity. But you don't see this in the level design, which is always 16x16 blocks (with the exception of the chozo statue, iirc). Note that this has little to do with the attribute grid since metroid is just using colour 0 for the far background field. There is
one case where the 8x8 fidelity doesn't work flawlessly: If samus is in morph-ball mode, and there is a ceiling right on top of the ball, and that ceiling is only 8 pixels thick, she will be able to un-morph through that ceiling and jump through it. I discovered that this was easily fixed by edititing a single line to no other effect, since that collision check was isolated to the unmorphing event.
I'm not sure what the reason for not having more 8x8 fidelity structures is (no real change in size, except you might be tempted to have more structures.. and which you could have since there was a
little bit of spare space left in that segment), but if the unmorphing bug was a reason, it'd be simpler to just fix it.
Then of course you have the sequence breaking exploit with the closing doors, which surely wasn't intended.
In Project Blue, we prevent the player from getting stuck in a wall if being transported into it by a transporting tile (conveyor belts or water streams). So no solid can be touching a transporter tile in the direction of transportation. It works okay in level 1-A and 1-B. In level 3-A and 3-B, you have the situation where i *want* a solid to meet the flow of water. This was fixed by making a lookalike tile of the flowing water that didn't have the transporting attribute set, so the player will stop being transported on the last running water tile that meets a solid. It's not as perfect as having code that can handle all these situations, but it works well enough.