I've played through Japan world yesterday, and man, audiovisually, it was an absolute treat! Amazing atmosphere and really awesome soundtrack. That's the kind of stuff I like to see in big collaboration hacks!
Quality-wise, though, this was the first world in my playthrough showing clear signs of typical "collab hack" issues. Some of those levels just didn't know when to end, taking the length of like three to four average Mario levels (and sometimes even three to four different themes). Often times with only a single checkpoint after like 5+ minutes of gameplay. I feel all of those should have definitely been split into separate levels (though of course from a technical perspective, that would be tricky). A bunch of levels also had rather janky gimmicks (a lot of issues related to hit-boxes that I noticed in that world). I'm glad I'm playing the hack in an environment with very little input latency, otherwise some of the levels might have driven me nuts. It's kinda telling when the castle at the end of a world is actually one of its easiest and most balanced levels.
These issues definitely make me emphasize the point I previously made about how SMWC coins should be commited immediately on acquisation. Couldn't even count the number of times I had to take that rather long detour in Pon-Pon Palace to get the SMWC coin right after the checkpoint. It feels rather annoying, because it's not even very challenging, and it never changes. There's no shortcuts, either. It really just adds half a minute of gameplay to every single attempt. Sure, there might be levels in the hack where surviving after collecting a coin is the real challenge, but I still feel this change would have more benefits than downsides. I'd say the hack should go full-on Donkey Kong Country in this regards.
I also went ahead and patched the tolerance timer patch to the ROM myself, and it seems to work without any issues, so I'd again like to propose applying it to future versions. I'm not actually sure how much of a difference it makes, since that's not exactly something that's easy to compare, especially when setting the frame counts to low values. Though I guess that's just another benefit of the patch, since that means it's unlikely to break any levels in the hack.
Feel free to visit my website/blog - it's updated rarely, but it looks pretty cool!