I'd make other builds, but I don't have Mac or Linux machines and HTML5 builds give an error...
If you have Godot, you can build it from source, though!
It doesn't realy matters if you have Mac or Linux to export the game to those platforms.
For example I use Manjaro Linux 64 and I exported my game for Win32, Win64, Linux32, Linux64, Mac and HTML5 I haven't tested it on any other platform apart from Linux64 and HTML5 but the testing of Godot's functionality is already tested.
Other from that, I have encountered a problem on the first boss. I killed everything but the game never allowed me to procceed.
Controls needs little bit more work. For example when you fire in the spaceship mode you cannot move up and down, another is that the spaceship does not face the dirrection of the movement, this last one seemed little bit wierd but helpful on the boss fight, so I do not know if it is a design decision or something else.
Another suggestion is to use in the display settings (in godot project properties) the stretch mode, put it on viewport and your game will get good scale on fulscreen. The way it is now, you can see all the level in fullscreen and etc.
Nice game overall a little rough around the edges but nice. It reminded me a little bit of Thexder, cause you can transform to a spaceship.
I'll see if I can export and test the other builds somehow.
Controls - You should be able to move in all directions when firing in ship mode. I also tested this on my laptop, so I believe the problem is keyboard ghosting. In a future update, I'm planning to add a toggle between firing in movement direction and also control changing to alleviate the problems. I'll take your word for the fullscreen tip too.
Thanks for the advice!