What type of microcontrollers are used in large scale UAVs?
Airbus used standard Intel 80186 and Motorola 68010 CPUs with 2.5MiB of memory in the FBW system for A320. Exactly the same chips that were used for computers at the time (around 1985) so they were not expensive and were already tested. Over time they upgraded to newer version of those CPUs. Each computer is composed from two boards with different CPUs, one produces the control output and the other verifies it and shuts the unit down if they disagree. The boards run different standard embedded operating systems and the software was written by two independent teams to minimize risk of the same failure mode in both boards.

Each board only does a little bit of work and the important units have backups. There are 4 units processing the sidestick input, 3 controlling elevator and ailerons, 3 controlling spoilers and elevator trim, IIRC 2 for yaw damper (FAC), 2 for each engine digital control, 1 for each display etc.

The requirements didn't really increase much since then. It is still desired that the tasks are split across many independent boards to minimize effects of one of them failing. So a CPU with few MHz clock rate and few MB of memory would still be able to do the job. But such slow chips are not manufactured anymore, so they are probably using whatever was most common chips few years ago.