First I want to congratulate the devs because this library is becoming the standard library for graphics. It’s awesome to see so many developments lately.
I want to jump to it and I’m searching for a recomendation for the best mcu to start with native GPU support. NXP or ST?
I think they are probably equivalent. The ST DMA2D engine has been supported in some form for a lot longer, so I would say it’s probably the more stable implementation. (Full disclosure: iIt’s also what I use. 
)
I don’t have any NXP hardware, so I can’t speak to the reliability of that implementation, but it was contributed by NXP itself, so it’s likely to be just as reliable.
Personally I would suggest basing your decision on other factors like display interface, RAM amounts, etc., as I doubt you will have an issue with either GPU.
I’ve responded on YouTube too but copy my answer here as well:
We have measured the ST GPU’s performance in several scenarios but haven’t found a significant change in performance compared to LVGL’s software rendering. The gain is rather in offloading the CPU. Unfortunately, we don’t have such deep measurements with NXP’s GPU yet.
I agree with @embeddedt, it’s very unlikely that there is a notable difference as the main limit is usually the memory bandwidth. There is no magic, the MCU needs to send a huge amount of data to fill the whole screen and it takes time.