CAVE/YMMV

Nightmare Fuel: Ultra mode in the Mushihime-sama series and second loops in Ketsui and the DoDonPachi series. Watching videos of these is so horrifying that many viewers get turned off from ever trying Bullet Hell shooters. It's for this reason that trying to introduce newcomers to danmaku shooters through extremely hard stuff is regarded as a bad idea.
- Even on Nico Nico Douga, many viewers (many of which can safely be assumed to be Touhou fans) cry out that the bosses are "cruel" or that they're "impossible."
Awesome Music: And you have veteran game music composer Manabu Namiki (who has also done work with Raizing, yet another shmup developer, as well as Konami's ReBirth lineup) to thank for a great plurality of it.
Complaining About Shows You Don't Watch: Most people who have heard of Cave shooters only know of their True Final Bosses, and as such are too terrified to even try any of the shooters on any difficulty.
Good Bad Bug: Pink Sweets has this in the form of infinite lives. It's used in order to crank up the rank so high that it makes Mushihime-sama look like a walk in the park. At least that would be the case, if not for the ability to cancel bullets...
Memetic Mutation: Dangun Feveron's unlockable ship, Uo Poko. AKA: MEOW MEOW MEOW MEOW MEOW MEOW MEOW MEOW MEOW
Polished Port: Espgaluda II's iPhone port is well-received, and is said by some to show that the iPhone is very capable of handling "hardcore" games.
Porting Disaster: Mushihime-sama and Ibara on PS2 suffer from reduced screen resolution and inaccurate slowdown; switching the screen mode to tate (vertical) mode reveals that these games aren't even 240x320. The Xbox 360 port of Dai Ou Jou and its Black Label rerelease not only have Loads and Loads of Loading, but stolen source code from the PS2 port, which was handled by a different company.
- Guwange on XBLA, at least if you're not playing on a widescreen TV. As many fans of shmups know, vertical-scrolling shooters typically have a horiztonal option that pillarboxes the screen. This wouldn't be so bad, except in Guwange, if you are using a screen with a 4:3 aspect ratio, the entire game screen, including the pillarboxes, will letterbox itself into 16:9. This means the screen becomes VERY small, and if you try to play with the monitor turned 90 degrees, you can't fill the entire screen with the game window due to the forced letterboxing.
Sequel Displacement: Espgaluda II, particularly in North America, thanks to the region-free Xbox 360 port and the iPhone port, the lattter of which was officially released in in the US.