Finally having some free time I got to really dig into my PCE Duo today. I am blown away by how many awesome shmups got left in Japan on it. With the vast majority of them being pretty damn good. I've also put about 2 - 3 hours into Book of Ys I+II today. Took me a few minutes to get used to the combat system but so far I'm really enjoying it.
There is something quite satisfying about ramming into enemies without the need to constantly mash an attack button. Although later on you will get magic rings that will help you with attacks.
Hope you get to try out the other pc engine falcom games especially Brandish, popful mail, Legend of Xanadu I & II and Dragon Slayers Legend of Heroes I and the other Ys games. I haven't had the chance yet to try out Sorcerian but it's on my backlog games to play.
I am currently working through Dragon Slayer legend of Heroes for Turbografx super CD.
If you can get around the weird viewpoint of Brandish you might find a great simple diablo-like dungeon crawler. That's one of my favorite pc engine falcom games.