Analogue machines lack flexibility of digital machines
Analogue devices can't make contingent if-then branches—ie analogue devices can't do "one thing under one set of circumstances and a completely different thing under a discretely different set of circumstances."
This limitation can be overcome by adding a discrete threshold element, but that still does not make an analogue device the best way to represent cognitive processes.
Zenon Pylyshyn (1974, p.68).