My take on the game.

Let me explain why it seems so hard.

Grapics:
The graphics aren't bad, but they aren't good either. As for the terms of the menu, there's really nothing to look at. Now, if it's just a WIP, then you know what to do. For in-game, Sonic Advance is my favorite selection. I personally think they look the best. So most of your stars come from that.

Sound:
There's really not much to comment. The music is okay, but it never really made me drawn to the stage like it should. However, sound isn't everything.

Gameplay:
I rated this one low because it's mainly based off the engine itself. You did however add in a SuperSonic feature, which feels miss-placed to me. However, I didn't get too far into the story yet so I won't bash on it.

Story:
Gave this five stars, because I only made it through the first level, so I'm not too sure what the game's about.

Interface:
This one is only low because as I stated before, the menu is practically nothing, and in-game, there isn't too much to look at either, but it's mainly the main menu.

Overall:
I don't think this is a bad game. It has alot of room for improvement, and you have a neck for level design. Just tune up your music, fix up your main menu, add in some other features that aren't on the engine, and you're in the right direction.

Also:

Some bugs I encountered. When I fell off the first stage, I never died. Just continued "falling" even though I was off-screen.

As for "Lack of some vital features," I meant that when you collected 100 rings, you didn't get a free life. I also haven't found any capsules that give you items. So there's some things to consider.

Best of luck on the development of your game.