I don't mind the mix of styles in the STM series so far, but that may be because of how I transitioned into it.
My first SMT game was SMT 3: Lucifer's Call/Nocturne. And in that one Kaneko did everything. The second SMT game I played was Persona 3. And here Soejima's art is what you'll look at for nearly 100% of the game when there's artwork on the screen. Not just because there are more characters and story than in a mainline SMT game, but also because unlike in mainline SMT, the demons are not part of the story really. They don't inhabit the world you live in, so you'll never encounter one like you can in mainline SMT games. They have no lines, except that small comment after you create them through fusion. You can fight them in battle in the "other world", but they'll be in 3D graphics. Not like in SMT IV where their sprites are their official artwork. There's no option to talk to them to persuade them to join you. And the ones who appear in the anime cutscenes are drawn in Soejima's style.
So the only time you look at Kaneko's work in P3 and P4 is in the menu. This separation of them from the story, having no personalities and lines (except after a fusion) make them seem much more impersonalized, and no different than the monsters in say Final Fantasy 10. There's clearly a big difference in art style between the characters and the monsters in Final Fantasy. But I don't feel like it clashes, since it's made clear that people and monsters are very different.
In SMT IV that's not really the case since the monsters are often treated as citizens, or equals you can converse with.
But when I played SMT IV, I wasn't bothered by that mix. Perhaps due to the separation of monsters and people in the Persona series, I've come to look at them like beings from another world, and that's why they look different, like monsters in many other rpg's. But I was just happy to see Kaneko's work again, while enjoying Doi's character design. It's whenever Nirasawa's abominations came into play that I got bothered.

-snip-

Yeah, his creations are not redesigned in a fundamental way when they appear in 3d. But the 3d models in P3 have a more cartoony style and are less detailed.



Btw, I want to note that I mistakenly remembered that the enemy sprites in SMT IV were just cropped out images of their official artwork. There was this enemy in the beginning of the game that looked like a blue horse with two heads. It was one of Nirasawa's abominations, and I remembered it like it was, perhaps because the artwork appears when you have a conversation with the monster. Another reason was because artwork of the enemies will often appear during boss battles.

Anyway, the monsters and characters in battle look fairly cartoony in P3. And Kaneko's distinct doll-like/human style faces for example on some of his humanoid characters are not particularly apparent in this style.
Actually, after thinking about this, I realized you may never actually fight any of Kanako's creations in Persona 3. And 4?
The monsters tend to look like this.

I may be remembering wrong, but I can't recall the monsters you face in battle ever being the Persona that you can acquire. In Persona 4 you face the Persona that your main characters use at one point or another, but I'm not sure who designed those.

So if that's the case, the only time you see Kaneko's monster design in P3 is when you use them as the main character, which causes a transparent ghost like image of them to appear above him for a brief moment when he shoots himself in the head with an evoker.

In the everyday life gameplay/story, you'll only ever see Soejima's artwork. And in battles, it's this cartoony style. In Menus you can see both Soejima and Kaneko's art. But along with the 3d cartoony, less detailed style in battle, and the separation of monsters from society, it made it feel like none of the designs were out of place.

It's a bit different when you see Doi's characters having a conversation with Kaneko's characters in SMT IV. But perhaps because of how I transitioned through the series, I didn't think about it as strange.

-

Well, Persona also started off being all Kaneko:



And that was appreciated. And then came Persona 3 with mixed artists for characters and Persona, and that was appreciated as well.
I agree that Kaneko's style feels more suited for a darker atmosphere. But I wonder if the mainline SMT series will go back to being all Kaneko. It may not. We're now seeing what appears to be yet another artist for SMT IV: Final.

Oh wow. I thought that only looked like her, because I noticed some slight differences, so I thought it was a different monster. But that actually is her. Looks like our prayers were heard then. That looks a lot less grotesque. Can't be an easy job to clean up some of his artwork though, like Lilith. Even if they make her look better, will she still have that giant gaping mouth, or will they remove things like that completely? I'm hoping for the latter.

And I found more great news on the SMT IV Final artwork.

This makes me more hopeful, because Nirasawa's artwork had a negative impact on the atmosphere for me.

Regarding Final, maybe there's been some miscommunication, but I said that there appears to be a new artist. At least to me, it doesn't look like Doi's work for the main characters any more. And then there's whoever's responsible for redesigning Nirasawa's artwork. Whether it's the main artist, Nirasawa himself, or someone else, these monsters are getting a different look.
But I agree it doesn't make much sense to change artists for a spinoff like this, so I'm not sure why they did that if that's the case. Unless Doi's work was also criticized like I assume Nirasawa's was. Or unless the game is very fundamentally different than the previous one.
I'd like to see an all Kaneko SMT game again. But I think they're going for mixed designs because they feel like some more 'contemporary' artists are a better look for the game as it's face outwards, for commercial purposes.

Must be, the US fans are still sleeping, though you'd expect there to be enough japanese fans to achieve this goal.

Or people are all bitter because of the P5 delay and want Atlus to finish one game before announcing another.

Oh yeah, I see now that he's listed under Character Design. That's good news for me. I'd rather have someone I already like that someone untested. (Thanks Nirasawa for making me worry about that.) I still don't recognise his style in those characters though. Then again I've only seen his work for SMT IV, the Persona 2 port for PSP, and Trauma Center, so I'm no expert.

It's good to hear that they're listening closely to feedback. About the demon conversations, thety keep making changes and improvements between every game, from my experience. But it's always tricky, so hopefully that part won't change all of a sudden.

As for Nocturne, a huge part of why I loved it was the atmosphere. It has one of the thickest atmospheres I've experienced, along with Majora's Mask. I'm not sure how that would hold up for me today though, considering the rather thin storytelling. The gameplay I think is ok compared to SMT IV. That you see all the characters on your team during combat, and the battle animations along with the very satisfying sound effect of Demifiend hitting the opponent, I feel like that part will hold up well in comparison. There's no 'Smirk' mechanic though to consider for strategy. But the fusion mechanics are terribly overcomplicated in comparison, and they make the game harder by only allowing you to learn each skill once for Demifiend, and only once. If you ditch it for another skill, you'll never be able to learn it again in any way.
I also really liked that Dante from DMC was in the game. But that was also during the height of the DMC hype.