But what's more off-putting is how it mixes tiny sprites with medium ones, giving the impression that the game is struggling to pull off 16-bit level sprites and action.
I've never played SAI so I can't comment on most of your criticism, but after watching a bit of youtubes I have to ask for clarification on this one point you make: can you share some examples of the offending sprites that make SAI a sub-16bitter, or direct me to the stuff I need to be smoking to see it?
I've not spent much time with NAI, either. So, again, I can't chime in on how the gameplay between to two compares, but to my eyes and ears, SAI is pretty amazing. NAI is impressive as well, but the limited (or shitty) parallax is a buzz kill, and a lot of the music makes my ears bleed.
SAI feels like Bonk NES. Lots of sprites look and feel too small, like it's a port to weaker hardware. Part of it is how everything feels while playing. My SNES loving friend rented it as soon as it came out and we played straight through it at his house that afternoon. We both felt the same, that it had a tech demo kind of coat of paint but felt awkward and unbalanced.
Those screen shots aren't great examples of what I was talking about, but already you can see that in SAI the bananas are the size of Higgins' head, while in NAI they are 3/4 the size of his body. The first stages of each look pretty clear to me, but playing makes it obvious. In NAI, Higgins is always running and he has decent animation and the games actually moves at a decent pace (still not as fast as I'd like though). In SAI, Higgins has poor and awkward animation, which is not timed correctly for what it is supposed to represent and how it reacts to your actions. At full speed, the entire game moves at the speed of a poorly optimized SNES launch game during slowdown. Higgins looks like a Western junior highschool student's notebook doodle. Many enemies are literally as tiny as they can be depicted by 240p pixel art and much of the stage layouts only draw attention to this imbalance.
If I can spare time when I am near a computer, I could throw together some example images. If my stuff was unpacked, it would be very easy to do a video comparison.
Even Wonderboy pulled off effective sense of speed and momentum with minimal animation. Good artwork helped a lot, but so did actual speed. If they were going to make SAI play like the anti-AI/Wonderboy game that it is, they shouldn't have attempted to do a more "realistic" player sprite with gangly limbs and a permanent expression of constipation. Swapping in the NAI sprite with all of its animation would really help, but it would all the obvious how floaty the jumps are and how slow the games moves.