Flawed
Ya the engines good and the graphics are flashy but there are some serious flaws.
For one, you lose all visibility in the direction you're moving. This is bad because you are most likely to be hit from the front. Please pan the camera so that you can see where you're going.
Another, enemies spawn way to close to you, and it gives you almost no time to react before they hit you. A lot of the times I was hit I didn't even see what it was that hit me because it spawned close in front of me and the camera didn't have time to catch up.
Third, the collision box should be slightly smaller for everything that can collide with you. I get "barely touched" a lot, and that should not count as a collision, especially when you always have to dodge so close to everything, if something brushes the edge of me it shouldn't count as a collision. You need to give some leeway there, it'll make the game feel much more fluent.
Fourth, I know the rising cost of health is "original" and all, but there's a reason why they didn't do it before. You end up punishing people for mistakes made while learning, and the price of healing 1 hit is tremendous compared to the amount you heal. If the heal healed you completely full then rose its price, that would be ok. But having it cost 100 pips to heal you 10% is ridiculous when coupled with the unfairness of the collision, camera, and spawn points.
Its not a bad engine but you need to do a lot more on polishing and balancing the gameplay, not just the graphics.