If you are hard real time, then the ability to country hose many cycles each assember instruction will take makes it easy to prove your code is correct. Run on a much faster modern 64 bit CPU with branch prediction, caches, and other tricks and there is no way to know how long any instruction will take. Getting non-real time things done is faster on the 64bit CPU, but the 8 bit is going to get the things it does done one time every time.