Posting since I started PCB/IaMP now (after getting sidetracked by other things)
PCB Thoughts
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The game has been fun to play through so far. It has been a bit on the slow side, just talking in terms of 1cc'ing the game as Stage 4 and 5 have been really rough to learn. Youmu is just tough fight that I can't really cheese/bomb my way through, though at least I've finally realized why I keep tripping up at the Prismrivers (I wasn't aware at first that the fight changed depending on which sister you damaged the most so that's improved things a little).
I mentioned how this game has a lot of streaming involved. I still think that it's the case. But I do think that ZUN has uses it soo much better in this game. There aren't those 8 directional homing shots that got used a lot by Patchy, Remilia and Flandre in certain nonspells. He's much better at making those parts less obvious. And the ones that are obvious, aren't trivialized by you knowing that they're stream-able (unlike EoSD's Stage 5), they're still tough and require you to be careful.
As for other gameplay related comments, I think this is the first game where the patterns are really refined. I do think that EoSD does a better job of individualizing each boss' bullet "theme" (like Meiling's rainbow bullets, Sakuya's knives and Remilia's red themed bullets) I do think the patterns are just overall better in PCB.
In the context of the series, this game introduces many mechanics such as
-showing the player hitbox and the enemy's position (these have been in every entry after this game)
-shot changing when you focus (something that's become prevalent again in recent games)
-bomb changing when you focus (also used in IN, can't remember if it's used in later games)
It also uses a sub-system (the Cherry scoring system), I believe spellcard history has been implemented with this and practice mode has been tweaked in this game (so now it functions like a proper practice mode).
Aside from that, the game is just a graphic improvement over EoSD. The ZUN art doesn't have a drop in quality as bad as EoSD (though it's still ZUN art). The backgrounds are much, much better though. Also I don't think there are any egregious color choices in this game (like most of Mystic Square, or Remilia's red bullets on red background fight) which is a plus.
In terms of music, I think this game is probably my absolute favorite overall soundtrack. Like there are tracks in the series I like better, but as far as consistency goes, I don't think anything really comes close. I think what I enjoy the most is that it gets the musical progression right; the songs just get better, there's nothing like IN stage 2's music (I like Deaf to All But the Song, and Song of the Night Sparrow, but they were absolutely wasted on a character like Mystia, and they had no business being Stage 2 themes only)
That's all I have to say right now on PCB.
As for IaMP I only really have first impressions. Unlike PCB which I've played before, I only really touched IaMP for a really short amount of time (I've played way more of Hisoutensoku and the newer fighters, and have actually cleared the newer ones)
IaMP First Impressions
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For someone that plays a lot of fighting games, it has been a pretty painful experience learning and adapting to this game. I don't mean that in a negative way; I don't think the game is terrible at all and it actually has its nuances. It's just that after playing so many fighters, it's hard to suddenly be expected to dash through projectiles to avoid them rather than block them, or to have distance based normals that change to projectiles (though I'm not new to distance based normals in general, since Vampire Savior had this for a few characters). It's just pretty rare for me to feel uncomfortable with a fighting game. This game has actually been pretty fun to play
I don't have any major thoughts so far since I'm still learning. I cannot say I'm a fan of the boss phases at all though. I thought I would like them on paper (it seems like a good way to let people learn the mechanics while making it clear that you're fighting a computer, not an actual human player) but they've just been kinda frustrating to be honest (but that opinion could change once I've properly adapted to the game). I also really like that there's no weather, or occult ball or popularity fluff.
I'm still on Reimu and I assume that using continues probably means I missed out on her proper ending, so I've still got quite a ways to go. But yeah, I've been playing a lot recently.
Also, I'm disappointed but happy that it's Reisen for PS4. I mean, I fail to see how Reisen is surprising, but if they do change up the next fighting game like them going from Hisoutensoku to Hopeless Masquerade, I don't have to feel bad about Reisen only appearing in a PS4 Port