I just made a really really quick micro-survey of flight controller literature and a few projects on github.
Seems SMC is only very marginally more complex than PID. The programs I looked at had something like 15 floating point values in the state and just a few dozen FP ops in each control cycle -- let's call it 100. Seems as if control cycles are run at 100 - 200 Hz. So you need on the order of 20,000 FLOPs, maximum.
A 16 MHz AVR with the standard Arduino FP libraries does around 100,000 FLOPs.
A lot of people seem to have moved on to ARM based boards. And then they write the software in freaking interpreted Python. Lol.
An Uno would seem to be a silly board to put on a quad. It's pretty big and heavy. You can get AVRs on much smaller boards. You can get *ARM* on much smaller boards.
Here's a photo I found on google of an Arduino Uno vs an older Teensy (looks like Teensy LC 48 MHz ARM Cortex M0)

The first Teensy's were AVR also. The latest Teensy 4.0 has a 600 MHz ARM with FPU and 1 MB RAM and it costs $20. It's really hard to see why you *wouldn't* use one if you need any kind of processing power at all. Even if you don't need speed, 1024 KB of RAM let's you run much more complex software than 2 KB of RAM.