Actually it is probably more secure than voting machines.

I have worked as a signature verifier for two elections in my state.

1. There is a physical ballot and signature.
2. For signature verification alone there are at least two people reviewing together each signature submitted with a ballot to match with multiple past signatures on file. The voting portion of the ballot is not seen by us, we only review the signature so there is no way to flag a signature based on how you vote. Any flagged signature goes up to further review by superiors.
3. Ballots with votes that are not crystal clear to the tally machines (if you put an x in the vote bubble instead of filling it out, if you used a pencil/colored pen instead of a blue or black pen, erasures, etc) are physically reviewed in person by a team of two people and if still uncertain flagged and sent to review by superiors.
4. Ballots put in the tally machines are manned by at least two people.
5. Cameras are placed throughout the workplace.
6. All ballots are locked behind chain link spaces when not in process.
7. The ballot processing stations are in a secure space open space, anyone can come and watch as we work. You just can’t get closer than about 8 to 12 feet of the work spaces cordoned off by rope.

The tldr, a lot of measures are in place to make sure everything is in the open, machines and people are double checking each other to prevent machine/human error/bias, and there is a evidence trail of paper/witnesses/logs/recording.