I could imagine a bad actor working at, say, a memory care facility could intercept the ballots, fill them out how he/she wishes, and get the memory care patients to sign them. I could imagine a domineering spouse or parent demanding that their spouse or kids vote a certain way and watching them fill out the ballot. I could imagine pay-for-vote schemes, where you can actually verify that the payee is voting the right way.
I’m not saying these are problems, but I could see them possibly causing problems.