Glad you enjoyed it!
We couldn't find a good balance for the i-frames, and ended up deciding to make it better for the player rather than leaving it too useless, so yeah, it is really strong. We'll try to balance it better.
The double jump is actually mentioned in the tutorial, but it's easy to miss. I think the coach goes "Remember you can double jump" or something like that, but there is not a panel like with the other controls, we'll work more in the tutorial, thanks for the feedback!