Mappa already used this style of animation (mixing "2D" with 3D) once when they animated Dorohedoro, they tend to use it when the work is very difficult to adapt due to the amount of details that are difficult to do by the traditional method. We should rather be thankful that Chainsaw Man looks a lot better than the horrible, horrible, horrible adaptation of Shingeki no Kyojin. You can compare the CGI between the two series, it looks better here.
By the way:
Some shots I looked at and considered whether they were just real photographs that had been heavily manipulated to give off the illusion of being drawn.
I'm surprised you mention this as a bad thing or as something strange, when it's a method of animation known as rotoscoping. Where they use real images and draw frame by frame on top of them to give a realistic effect.
Yeah Mappa has lost me for a while now honestly. I couldn't finish Dorohedoro either. I was the same in regards to Love, Death & Robots when it came to the realistic/3D episodes, I can't really enjoy animation that looks like it's from a video game -- I should add I also don't even really play games anymore so I'm even more put off by it.
I wasn't sure if it was rotoscoping or just very strange art when it came to the backgrounds. Makes sense for the moments of action where it was definitely more evident.
I don't think I'll end up sticking around to watch it, I might check out the second episode but this style definitely just is not my thing.
I'm pretty far behind having dropped that years ago; needing to check back in on it but I did see clips from it. That wasn't pleasing to the eye at all.