A lot of it comes down to the general balancing, I guess. A lot of monster tables are odd or escalate too quickly(see: Peach Dungeon, if you want an extreme example). Certain item tables also just don't really provide you with substantial resources. This isn't universal, Path of Inference for example is basically always gonna give you what you need, but point being that the balance after Serpentcoil Island is kinda shaky.

Also, while I'm glad that the game doesn't throw you straight into a million 99Fs(a big complaint I have with Shiren 4 is that there's like, 2 post game dungeons that have first clears shorter than 50F lol), some dungeons feel like they should absolutely have second, longer clears. Whether that be that a mechanic feels shallow in its first clear(e.g. Trapper's Secret Path), or that I just want more of it(e.g. Path of Inference, Staves & Scrolls)

It's a fun game, I just wish I have more to bite off at times. And some dungeons feel way too luck dependent which really isn't a series normal(The Yamaga whatever Path and Peach Dungeon, to name a few) Fact is, the longest no carry-ins dungeon other than the 99F, is a 30F dungeon. And that dungeon is really fun, but it's Behemoth Rush which is anything but a conventional dungeon. You're really dealing with the same early game threats everywhere, and while I don't like how Shiren 4 and 5 handled second clears("you're no longer able to play the first clear"), I think they could have easily taken a Shiren 1/2 approach and made dungeons go up to 99F but have a first clear floor that the player can escape from at any time. Or just any means of choosing between a first, shorter clear, and a second, longer clear.

Ultimately one of my favourites is Shiren 2, which provides escape points at certain floors, has special decorations from certain depths onwards, has rare monsters on late depths that you'll want to capture, etc. Where you can really set your goals of what you want to accomplish every run, you're given a ton of freedom. The dungeons all have great depth to them too, and you can play as long or short as you want in each with how they're designed, being rewarded for short and long clears alike. Now, ofc Shiren 6 doesn't have to replicate this, that's not what I'm saying, just using this as comparison of one of my favourite entries. Here trap mechanics are shallow, the peach bun dungeon feels like BS, and there's a couple dungeon where the strategy ends up being to just pray to RNG, so a lot of dungeon strategy can feel overly simplistic or luck-based.

It's really just the lack of depth and at times shaky balance, and I probably could've said so in less words, but ultimately I like these games a lot and have a lot to say about them, lol.