I don't think the issue is SW for voting machines per say, as much as it is the typical SW developer mindset does not lend itself to developing voting machines that should be used.

To give an example, lets say we were to split the voting machine in two parts (for some reason). Which of the following would make sense to use for the physical link between the two:

1. USB 3.0
2. 1Gbps Ethernet
3. RS232

Conventional logic would dictate ethernet as it is fairly easy to set up something on both sides to talk back and forth. Next would be USB 3.0 since it is commonly available. I would argue RS232 is better as it has a limited data rate (<20000 bits per second). If someone compromises one side of the machine, the likelihood of successfully compromising the other side is lower due to the much lower data rate.

(To be clear: this is the opinion of someone who uses UDP because TCP is too slow, so take it with a grain of salt).