There are two tricky parts that come with allowing prisoners to vote that must be considered. Not hard stops, but just additional dynamics that will be in place.

1. Prisoners have little to no autonomy, and can therefore be easily coerced into voting a certain way. If the warden/prison staff lean conservative and they hear that a certain prisoner voted liberal, that prisoner is vulnerable to reprisal. There would need to be an additional entity present in prisons to enforce privacy of voting results. But how do we guarantee that this government entity won't just collude with the other government entity running the prison?

2. There may be problems in terms of where these votes are counted for. One way to protect the anonymity of prison votes is to pool them among the district that houses the prison. But do we let the prisoners vote for local candidates/laws when they are not locals? In many cases, prisons are located in very small towns and may therefore significantly skew local elections if they participate in them. So does everyone get an absentee ballot for their place of origin instead? Even if the duration of their sentence means they are likely never to go back there? Or do prisoners only get to vote on items/candidates at the federal level?