I just played through the demo, seems like you have a solid foundation here to build off of.
As far as the gameplay is concerned it reminded me of Contra, I didn't find any noticeable bugs and overall it felt pretty good to play.
One thing I'd suggest looking into is the custom controls implementation and UI. It wasn't terribly difficult to figure out but I think it could be presented in an easier to understand way. In addition, I noticed that the custom controls doesn't seem to have any sort of error checking present, so you are able to bind a single button to multiple commands.
Besides that, good job!