Ah, what a day to see this game finally marked abandoned. I want to say "good riddance," but that's not quite right. Game played like shit, had a few too many fetishes or enemy designs I didn't like, and was generally pretty bad... but at the same time it was kind of the most prominent Sonic-themed ero-game, and certainly more tolerable than pedoshit like Babysitting Cream (ignoring the pedoshit in this game, that is.)
Feels like the concept of a Sonic ero-game is difficult to get right for multiple reasons. Can't have it play too similarly to just about anything in the actual games without completely breaking the speed & flow that they're known for.
(Disregarding entries like Sonic Battle which is different but still very fun and managing to have just enough influence from the main games. And also disregarding stuff like Sonic Chronicles which is just bad.)
Then if you go for something different, you'll probably get complaints no matter what. Especially if you were to just try and make it simple with a visual novel or something similar, you'd get inevitable complaints about how "overused" the format is or "wahhh why isn't this porn game super ultra exciting, I could just play an actual game for good gameplay but instead I gotta complain about this."
Then another note you'd have to deal with what kind of porn game it'd be. Lots of different approaches there. Playing as a male character like Sonic, or something/someone easy to "project" unto like a more-generic character, and having sex with different characters. Could be straight, gay, mix of both, depends on who's making it.
[Playing as a female character and seeing what kind of different sexy scenarios she ends up in] is what this game (mostly) focused on, and it's a popular-enough format. Not personally my favorite considering the types of things/characters/etc. fucking a female MC tend to be... unappealing or ugly.
Then you also have to narrow those down. Are you going to include lesbian scenes in your female-focused game? How much lesbian stuff? Is it good lesbian stuff? What about dickgirl content? Not for everyone but certainly popular. Hell, just about any fetish or kink is going to affect the interest in a game.
Anyways, rant over. I'm sure there's more I could talk about, like 2D vs. 3D gameplay and/or graphics but I've already typed up a storm here and I really don't want to waste any more brainpower on this shit.
EDIT:
Always thought it was strange that this game never really "advertised" the kind of fetish/kink content that was included. Everything up-front about the game was always just "scientist OCs accidentally cause horny pandemic in the Sonic setting" and that was it. But then you actually play the game and you're greeted with... stuff that goes beyond your typical "if you get caught by an enemy you see sexy animation" games.
The cum-inflation with belly-button popping, all-the-way-through insertions, succubus & incubus enemies that cause genderbend animations, and that... really fucking weird and unpleasant birthing one with the ghost girl. Hell, the very first enemy you encounter literally has your character getting sexually penetrated by a sharp drill. To bring up the aforementioned "things fucking girl MCs tend to be unappealing or ugly," I really don't want to see goddamn Grounder in what's supposed to be an erotic context, especially with his existence in the Sonic cartoon as a bumbling comic-relief idiot. (And then the first boss fight is the trio of comic-relief idiots from Sonic X, just great.)
Some of the animations just felt like they never matched up with the enemy or character, either. The mermaids in particular stand out as a huge disappointment to me, I expected something better than... forced tail insertion. The moto bugs in the first stage? "Ehh we'll just have tentacles come out of its back, that makes sense!"
