Pretty good tutorial, but it's very easy to skip forward without reading what's written. I played tutorial 2 times because something I was doing caused tutorial to progress, while I wasn't done with reading the text.
Some bullets are hard to see on some backgrounds. For example at 8:44 the background is very bright, in shades of yellows and greens, same as enemy bullets. On some spots bullets almost disappear. Maybe each alien type could have a list of backgrounds that work well with their color to not change them?
Lack of clear info at the start. First I get to pick a country (where each has a different bonus) and then select one of the planes, which are also grouped by country. Do they also have country bonus?
There should be info about stats of the planes. It's obvious each has different speed and guns, but do they also have different hp and ramming damage?
I didn't feel compelled to use rockets or bombs. I got used to machine guns and decided to just stick with them.
Enemy numbers and patterns are mostly ok, but 2 times I got so swarmed with blue aliens I feel like it's impossible to kill them and dodge all the bullets.
That boss fight was pretty good, I liked it.