A pretty common downside is a lack of adherence to historical boundaries and groupings - some neighborhoods that legitimately formed a district and a unified block due to strange geographic features could be broken up.
But actually - there are two classes of algorithms out there: one can determine how to fairly divide up a state and another measures how unfairly a state is divided up - instead of going full into computers-determine-everything it might be nice to just have a requirement that the result of the election in the state is relatively statistically probable given the proportions of votes for each party. So people could still draw districts we would just have a manner to prove their districts are acceptably fair.
People actually will have to tune into politics since the differences will be diminished and you can't simply push the red or blue button and be done with it. 'aint nobody got time for that!
1. There would be a lot of sudden changes, without any real explanation about those particular changes.
2. Unlike now there would be little on-the-ground reasoning to these changes. Districts would be split on random-seeming streets, with no respect at all about residents felt. Now at least you have someone to blame, and we like to have someone to blame rather than "the computer".
But actually - there are two classes of algorithms out there: one can determine how to fairly divide up a state and another measures how unfairly a state is divided up - instead of going full into computers-determine-everything it might be nice to just have a requirement that the result of the election in the state is relatively statistically probable given the proportions of votes for each party. So people could still draw districts we would just have a manner to prove their districts are acceptably fair.