I've had one of at least every Umihara Kawase physical copy, so I've had this one too. Good thing I got it for a good price and don't have it anymore. It really does get wrong the main thing it shouldn't get wrong- the physics. Honestly, one of the most bothersome things about it to me was the perspective- it changes the perfectly good one from the First and Second Editions to a vastly less appealing and more nonsensical fake 3D perspective like in NES games. I guess I'm not too upset because Shun is my least favorite in the series (and it will probably remain that way) and has enough good remakes already. They made up for it pretty fast with the DS one a year later. I'd actually say the First Edition is probably my favorite version of that game. At least its flaws and quirks (such as the cutscenes and the music suspension before loading) are sort of endearing. Plus it's missing some irritatingly designed fields from later versions.
I still maintain my position that Shun Portable isn't the only bad game to blight the otherwise perceptively spotless series- I contend that Sayonara 3DS is arguably an equally obsolete and flawed version of that game, somewhat made up for by the fact that it's just exciting to have a new Umihara game in the modern age and it was the first version of it that came out, quickly superceded by its vastly superior remake, rather than the third that makes you wonder what happened over the past eight years. The framerate was choppy, the game looked drab and ugly almost to the point of creepiness, levels felt incomplete and devoid of enemies, and then there was that awful puffer fish boss made much better in the remake. Sayonara 3DS deserves to be buried just like Shun Portable. And there's just little flaws too like when you restart a level, the music also restarts and doesn't seamlessly flow like every other game in the series does all the time.
And if there's any "bad" version of Original, it's the DS one, made up for by the fact that it was included as an extra of Shun DS (the definitive version of that game at the time) and didn't have many problems other than minor nitpicks like the music being worse, the screen being smaller (what could they do about that?) bosses not picking up speed and being extra long and boring (already a problem with the game in general that presents a huge roadblock for beginners) and the fact that there was still no stage select like in the 2015 version. Still, it's passable for someone who wanted to play Original Umihara on a handheld prior to 2015 (I'm in that group). It even had arguable improvements over the original like better loading times and a more precise timer. There are also preferences like wanting the Taki and Keiryu songs to be where they were in the Super Famicom version, but over all, it's no big deal to most people.