IMHO, Fredkin picked cellular automata for good reason. Regularity helps explain how light travels in straight lines across long distances. There /are/ asynchronous CAs, but asynchrony is mostly an unneeded complication in this context - synchronous CAs are hard enough to work with, thank you. CAs can be universal, so they can do anything any other discrete model can. CAs are low-level modelling tools, that are not easy to build things with - but it seems extremely likely that low-level modelling tools will ultimately be needed to explain physics.