A lot harder to count, a lot more errors, and much easier to hack (replace ballots with fakes filled out the way you want them.) They're also legally problematic because they are not disabled-friendly.
Harder to count - yes. So what?
More errors? That is debatable.
Easy to hack? Paper - certainly possible but requires immense work. More work than getting it right the first time. Electronic machines - There are plenty of wicked puppies on the dark web who could hack your machines far easier than ballot stuffing.
Disabled friendly? Our paper system work quite well. If problems are found we fix them instead of complaining. O
h wait I see your problem now.
No ballot system will ever be 100% error free or easy. But how much is your electoral process worth to you?
The problem is not the electoral process per se. Rather the problem(s) is that you Yanks are whingers, hate each other so much and do not like to wait.