I've played Forbidden Siren 1 through to completion and Siren: Blood Curse. This topic's timing is actually quite the timing for me, as I literally decided two days ago one of the games I was going to finally play through and complete during the summer was going to be Forbidden Siren 2.
The games are very unique, and the first game earns it's position as being one of the hardest horror games ever. But I think there's a specific type of logic to most of the game, three separate revelations you sorta' have where you begin to progress in the game much faster than before. The first is when you break out of the first "loop" and gain access to the time table/level select, which is pretty early in the experience as a whole but does really change things. The second time is when the player begins to sorta' understand how the game's internal logic works, missions and objectives that earlier were such struggles for you begin to become cakewalks and you begin to sorta' figure out the logic behind the primary objectives, like if the objective is to stop a certain Shibito, you begin to learn which Shibito have special behaviors to stick out to stop, or if there's a mission with a character you need to escort yo begin to learn where often the "safe spots" to leave AI partners are while you go off to do what you need to do, and how to hurry them past areas before the AI regenerates. Finally, there's a revelation where you begin to learn how to do the hidden "side objectives", how to figure out what levels have hidden objectives and the sorta' logic to how to break time loops and unlock new objectives/levels by changing outcomes in earlier levels.
I say there's a specific type of logic MOST of the time, but there are a few moments of complete bullshit in the original game. The thing that crops up into my mind almost immediately is the lantern quest, where there's four lanterns you need to light in four different levels in the game to progress. But there's four big issues with this quest:
01. The game does not telegraph well at all that lighting the lanterns has special purpose to progress in the game.
02. There's no clue to what levels have the lanterns you need.
03. There's often more than one lantern hidden away in the levels you do need to light them in, and only one that looks slightly different will even interact if you try to light it.
04. Frankly the problem proposed for this quest, lighting all the lanterns is a nonsense adventure game logic solution which breaks away from the game's internal logic most of the time.
I think that's just badly designed and telegraphed so it's not fair to the game's logic which is present most of the time. But most of the time there is a sorta' "flow" to how the game works, and as you play more and begin to understand the game's internal workings, you begin to progress much faster and perform much better in the game, even in new levels with twists and such to the formula.
I will say I think the design goes a bit against the horror aspect of the game though. Forbidden Siren has legitimately creepy enemies, settings, atmosphere, etc., so it's a bit of a shame that this is kinda' sacrificed by you having to then understand every bit of the game to progress successfully. I also think this is why Siren: Blood Curse in some ways manages to be scarier (in my opinion) than the original Siren, because though the game is easier and a bit westernized in some ways (though westernized in the sense it's still Japanese developers trying to westernize the game so there's a bit of discord that's enjoyable), because the game doesn't require as much mastery to progress while still being challenging (but not as challenging), you don't need to figure out everything about the game to progress which allows there to be potentially more tension going through as you are allowed to fumble more rather than rationalize and need to understand everything to progress.
I suspect Forbidden Siren 2 will end up being my favorite game in the franchise. I love the setting and enemies within it. I will see when I finally go through with it pretty soon, I've heard while the game is still challenging it is less so than the original Siren, and I also have heard it has quite a story. I think the setting and story of Siren 1 and Blood Curse are both big triumphs of the game's and made both very interesting to go through all the way, along with the Shibito themselves. I think these elements do remain strong even when you begin to figure out how the game ticks most of the time, so it does hold the experience in the later parts for those committed to seeing it through. And I think that aspect will hold a lot of appeal for me personally in the sequel as I kinda' love the setting of the sequel from what I've seen, and heard good things on the other aspects.
Also, as an aside, I'm really happy fans have gotten to calling the series Forbidden Siren. It was only called that in Europe, and Siren is cool too, but I think Forbidden Siren is just such a cool title for a horror game series.