First, you've omitted a major downside of digital vs. paper systems, scalability. Attacks on paper systems are much harder to scale, are more easily detected and more readily audited by staff who need no IT training/experience. There are other downsides, but scalability is a good example.
Second, the "plenty of upsides" are mere niceties. I don't know of any must-haves unique to digital voting systems. What do they offer that is worth their diminished security compared with paper ballots?
I put security of our electoral process at top priority. Given the difference in attack surfaces of digital systems vs. paper ballots, I can't see a case for the former.
The continuing parade of breaches of supposedly super secure systems indicates we don't yet know how to code systems for large-scale public applications where security is absolutely essential. Paper ballots are the best we have for now.