Just being mentioned in the same sentence as Metal Slug is a massive compliment for us! Thanks for the suggestions. We are balancing the difficulty for our next release. The major issue with the spikes was that it used a 16x16 collider which took up the entire tile. We initially used a tilemap for the spikes and used the default collision box (which cannot be edited). We fixed that and are now using a custom collision shape that aligns with the shape of the spikes and is more forgiving.
We are now exploring either the use of a health bar and/or the addition of checkpoints. I want the game to be challenging and to give the player the feeling of accomplishment when they pass a difficult area. So, we are leaning more towards adding checkpoints (it's actually the most requested feature among our players).
Thanks again for playing our game!