Biggest issue is surely speed?

Computers are measured in MHz and GHz. How do you even get close to that using mechanical means?

Speed is also a core value proposition (to use contemporary parlance) of computers I.e computers can carry out calculations at a rate of MHz/Ghz. If you want to talk in any meaningful sense of “computation”, and the usefulness thereof, speed of computation is a key metric.

I reckon cooling would be an even bigger concern than it is currently.