While I'm probably better than average at shmups, I don't consider myself an expert by any means (I've never touched the touhou games), and the difficulty felt fairly low. After looping through level 5 a few times I dropped out at around 80k points, but at that point it felt pretty rote.
I think if the difficulty ramped up faster or farther, it would be a better experience, but as it is, it feels too slow and simple. There is decent enemy variety, but the last level doesn't use all of them, and since that's the one that repeats, it feels stale fairly quickly. The last level is missing the missile enemy, which is one of the more challenging ones, I think.
Another issue with the difficulty is that the powerup is *very* strong and *very* easy to acquire, leading to basically no threat to a decent shmup player who doesn't use them constantly. I began using them basically every time I felt at all threatened, and still had a stock of over 20 of them.
That said, ramping up the enemy variety and difficulty farther would quickly improve the experience, in my opinion. Those aspects are primarily what's holding it back.
Also, the keyboard controls are very awkward. WASD to move with Z and X for shooting and powerups makes you stack your hands on top of each other, or else make you contort your left hand in bizarre ways if you want to only use that hand. I had to plug in my controller to play.