My S23U is definitely bright enough in most conditions, but is still barely visible on a very sunny day - even with the awful colour boosted mode kicking in.
I'm kind of surprised to hear that. Your S23 Ultra has the same display as my S22 Plus. I was in the desert in Moab in the middle of summer and still had no issue using my phone for hiking maps and etc...
Galaxy S24 series will share the same peak brightness of 2,500 nits
That's compared to the P8P's 2400. But again, both are peaks, not full-screen brightness.
According to https://www.devicespecifications.com/en/editor-review/22c0e9/3 , the P8P can maintain full screen brightness at about 1450 nits, which is also very very bright. I mean, it wasn't long ago that 800 nits was considered "good". My Pixel 2 topped out at 400 IIRC (it was bad; hardly visible in sunlight). Hopefully we are nearly past the point where this is a differentiator among high-end phones.