There is one single very dangerous problem with electronic voting: Trust. People have to trust it, because they are unable to test it.
With paper and pen, it's easy. You can nominate anyone to work as an election monitor. The necessary qualification is "being able to find out where the X marks the spot" and "count". That's a skill set available to nearly everyone.
Working as an election monitor to rule out foul play with election machines requires someone to know quite a bit about computers. It's anything BUT simple to rule out foul play.
The danger here isn't even so much that manipulation can take place. And I don't even want to engage in the discussion whether or not these machines can easily be manipulated. The danger is that some populist aiming for the uneducated masses goes and cries foul play when he loses the election. And that's a danger not to some party but to the faith of the population in the whole democratic process. And that inherently is dangerous to democracy altogether.
It's not easy to debunk such claims. With paper, it's easy to go "oh please, count them yourself if you don't believe us. Here's the paper slips, and you can count, can't you?". Now try the same with election machines. Saying "you can do an audit yourself" isn't going to cut it. Why should we trust the computer experts? It's not something just anyone can do.
These machines are a danger to democracy. Nothing less.