things generally in the same family as an arduino, such an STM32F7 based flight controller for a hobby UAV, can have features like eight UARTs and other things not found on a raspberry pi. and also in a much smaller form factor. There are some that measure 20x20mm.

https://www.google.com/search?client=firefox-b-1-d&q=stm32f7...

https://oscarliang.com/f1-f3-f4-flight-controller/

for instance: http://www.mateksys.com/?portfolio=f722-se