The belt's designed to be easily compiled to (register coloring is NP hard) but it requires the CPU analyse data flow somewhat to optimize in a way where there isn't a bunch of data copying. Godard frequently explains that the belt is conceptual, that the hardware should be doing optimizations underneath

Whereas for a simple architecture registers are simple: they index into the register file. This only breaks down when we bring in out-of-order & register renaming in order to have more registers than the instruction set specifies