A raspberry pi is going to be slower than a modern computer these days with over 4gb of ram and a modern operating system. But the good thing about the raspberry pi is it's pretty nippy and doesn't need a lot of storage space for the operating system.
Well if he wants a raspberry pi to run brainflayer he may just be looking forward to bruteforcing brainwallets, so he would need to calculate the amount of electricity consumption in relation to the computing power and thus chances of breaking into an existing seed. So hey may be better of staking raspberry pi's than using fewer faster computers.
If you dont know what brainflayer is:
https://rya.nc/cracking_cryptocurrency_brainwallets.pdf
This is why you never use wallets that can be spawned with seeds.
