VideoGame A series held back by glaring flaws
There is a reason Seihou never became massively popular. Objectively seen, Touhou Project is overall better, and only the Touhou cameos gave Seihou anything resembling popularity. That being said, I believe that you should give Seihou a shot, if only for the music.
Shuusou Gyoku
Shuusou Gyoku is, objectively seen, decent at least. It has high points, such as the Stage 2 boss fight, which is easy, but still a blast to play. It has low points, such as over-reliance on VIVIT's very tiny hitbox and terrible graphical presentation at parts (bullets hard to make out), as well as really damn bad sound effects. Some boss patterns are also rather uninspired, and not that many bosses overall have a theme of what they are doing. Milia uses fanning laser spreads and restrictive bullet chains. Mei&Mai use bullrushing attacks, sweeping lasers and cover-fire. But what's Gates do? You sometimes miss the "signature" moves, which only were codified by Kioh Gyoku. Speaking of which...
Kioh Gyoku
This is better than Phantasmagoria of Flower View. The balance is not perfect but there are no god-tier characters (Shiki/Komachi hurrrr), the basic game mechanics, particularily the emphasis on grazing, are well thought-out and the game plays smoothly. Characters are well-defined, have their roles, have distinctive signature moves (I could say what every character does in Kioh... I can't remember most of Po FV's stuff.) So overall, this is the most well-designed Seihou game. But all that will probably not matter much if you are restricted to single-player. Story mode is a joke with a rather fixed chain of five bosses and only the last two matter plus three unlockable characters - that's four playthroughs, which probably won't take you more than two hours if you are not stalemated by blatantly shieldhacking Muse. But assuming you can set up Netplay, you will probably have a blast.
Banshiryuu
C67 is blatantly unfinished but shows promise, with great stage 1, 2 and 5 boss fights and overall well-designed stages. Then the C74 version throws all of that out of the window for ooh shiny graphics. You still should give C67 a shot, but you better never play C74 yourself, just watch the extra stage playthroughs.
I personally have a special connection to Shuusou Gyoku. It may not be rational, but I still enjoy it.
VideoGame Three games, none of them of them exceptional
The games are all quite different and probably deserve seperate reviews, but they're simple enough that there's not much to say
Shuusou Gyoku: A simple vertical danmaku shooter. It is by no means a bad game. It's also not really a good game. There are no twists to the gameplay, and the patterns themselves are all pretty standard. It's only worth playing if you really need a danmaku fix or you want to fight the Touhou cameos
Kioh Gyoku: Kioh Gyoku is a versus shooter similar to Twinkle Star Sprites or Touhou's Phantasmagoria games. In many ways, it's quite good, with the use of graze to gain power for attacks in particular making it based more on skill, helped by a generally lower level of randomness than the others. It still has a number of problems, though. Most obviously, it's not terribly well balanced, even discounting the boss characters. The regenerating shield mechanic also leads to a number of irritating oddities, like going from full to one hit from death in under a second, or having your opponent repeatedly come back from the brink. Additionally, the single-player is quite awful, with terrible AI, a cheating Final Boss, and no difficulty levels. You'll need to pester someone into playing with you if you want to get any real use out of it
Banshiryuu: A return to more typical danmaku. It was initially released in a not quite completed version at Comiket 67. The "completed" version released at C74 was closer to a remake, featuring new graphics, altered mechanics, and replacing a lot of the patterns. However, the main issue I have with Banshiryuu remains the same in both versions: It's a perfect example of style over substance. The game is quite flashy from start to finish, but only rarely is it fun to play. The patterns are typically unfair, somewhat boring, or both. It is better experienced by watching someone play it than by actually playing it
In short, I'm not really sad that this series has died
VideoGame Not a Perfect Series, But Still Pretty Damn Fun
Shuusou Gyoku The first game in the series, and my Personal Favorite. Yes, the graphics are crap and the music is on outdated MIDI hardware. However, ZUN's amazing musical gift makes every song memorable and catchy, and the crappy graphics have a sort of charm to them. The gameplay is still fun, with a wide variety of shot types and boss patterns. And hey, we have cameos from Reimu and Marisa in a fantastic (though horrifically difficult) boss fight. All of the characters are very fun and memorable too! Shuusou Gyoku is a great game to check out if you want to play a fun little shmup.
Kioh Gyoku While the A.I. is terrible, the game is still really fun. It's very quick, meaning it's easy to pick it up and play in short bursts. Not to mention, it's great for scoring. The graphics aren't too bad, and once again, ZUN's music is just brilliant. It's even the first time he ever used his signature ZU Npets! I find Kioh Gyoku better than the two Touhou VS. Shooters (and I haven't played Twinkle Star Sprites), because of how quick and fun it is. Check it out!
Banshiryuu This game is pretty divisive. People who like it are often scorers, and people hate it because ZUN wasn't involved. Even though ZUN didn't compose the music, the music is still wonderful! There are two different versions of the game, so it's even more confusing to talk about. The C67 version is EXTREMELY difficult. There are few resources and some shocking patterns. However, most of the patterns are still fair, but require TONS of memorization. The C74 version is a little bit easier, because you get so many bombs. However, the patterns are a lot more unfair, so it's not as enjoyable. The main reason why I like the C74 version is the extra stage. Good lord, it's amazing. It's a crazy boss rush of all of the characters from the first two games! All in all, Banshiryuu is kind of mediocre. I would recommend the C67 for the main story, but the C74 only for the extra. Check out both, and see which one you prefer.
Seihou is pretty awesome, and I really wish it would be continued.