Every actual study of the matter I've seen or heard of has shown no evidence of any significant fraudulent voting, particularly not systemic fraudulent voting. While there might be a theoretical concern about incentives here, there's a lot of problems we know are happening right now with voter suppression under the system that we have.
Machine politics and the nonsense behind it is a form of vote fraud. While southern states have a long history of denying African Americans the ability to vote, states where county politics controls voting like New York are also problematic.
More directly, there are plenty of schemes to fraudulently get votes counted, from absentee ballot fraud in nursing homes to voting irregularities in religious communities.
California recently had to be forced by lawsuit to remove 5 million invalid voter registrations. Multiple counties have more registered voters than they have age 18+ citizens. It is unclear how often these registrations were actually used to vote, but isn't it possible that ballots could be printed and filled surreptitiously for these registrations in the future?
https://www.realclearpolitics.com/articles/2019/06/20/calif_...