Personally, my concern is that with mail in ballots some nutjob that believes there's ballot stuffing can set fire to the ballotbox and even though they're caught it's a major inconvenience to get a replacement ballot and the websites that show your ballot is received take days to update.
But I still love mail in voting. My state sends a candidate brochure with it and I can take my time to actually look up all those random candidates' policies. It takes me hours to actually fill out my ballot but that's a feature, not a bug (there's nothing preventing you from along party lines but frankly I'd be happier without parties)