Something that immediately stood out to me before I even started playing: your pixel sizes are all over the place. See the pixel sizes of the playable character versus the clouds and background.

I don't see why you have the sprite transform when the player jumps. You're better off doing squash-stretch animation for this type of character. 

The controls feel awkward. A/D and Space on one hand is uncomfortable, and I found myself wishing that A was my jump key instead of space while using the arrow keys.

The problem with your controls is how fast you accelerate and how slippery the character feels once you hit the ground. It feels very difficult to control the character, and because you made the platforms at the start of the game so small it winds up just feeling really frustrating. You want to ease the player into your game first before ramping up the challenge, unless your intention was to make it a rage platformer-type game.

No matter where you are you're always at risk of falling all the way down to the very start. This is bad design for the very first level of the game - you want players to become hooked by the game, not quit out of frustration.

Why not make the levels more horizontal and add challenge that way? You've got the character set up to go really fast, which makes it feel like its potential is wasted jumping up platforms on a narrow stage.

I notice that I could get stuck on the very corner pixels and keep rolling in place until I jumped. Also, do you have coyote time implemented?

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Your choice of engine isn't going to change whether your game is good or not. Find one that fits the needs of your project and use it to its potential. Unreal has a pretty steep learning curve, and it's usually used for 3D games. Since you're making a 2D game, I'd stick with an engine that is better suited for it if your plan is to continue on that path, which in your case sounds like Godot.

Godot. I think your game has a lot of potential. Good luck :)