Very quickly, I'm not speaking to the Ballot Counting System - I'm speaking to the Ballot Request System. The Request requires information that could be obtained from an individual and rerouted to a centralized operation to fill in the ballot for the person.
It is akin to what happens in some states where some folks are bussed to the polls and given a ballot pre-filled by a trusted member in their community and told to follow the ballot template and we will all go get dinner afterward.
However, in the case of the mail in ballot, the physical voter is no longer required, leveraging the efforts of one person beyond the previous systems limitations: i.e. bus size, trip time, etc.
^ Real life example of the previous exploit -- that is legal if done correctly.
Back to the point: the BRS had an implicit control in place previously... as in, if we see a ton of mail in requests in one location, we should probably check that out. Now that control is invalidated because of the pandemic... thus removing an implicit control without recreating a control to achieve parity (the recreated control for me is the queries I laid out above).