Two different kinds of missing
There is a difference between a transaction you can see but cannot classify and a document that does not exist in your world at all. The first is a question: what was this $840 payment to a supply house, and does it belong to the job you billed in July. The second is an absence: there is no statement for the second card, so an entire month of activity is invisible and no amount of reviewing will surface it.
Most practices carry both, and they weigh differently by season. In a steady month the ledger questions dominate. In January, in a catch up engagement, or in the fortnight before the partnership deadline, the absences dominate and they are the ones that stop a close cold. Buy against the season that hurts you most, because that is where the hours actually go.
Why the chase has to start before the posting
A bookkeeper who waits until posting begins to notice what is missing has already lost the useful part of the month. The client had three weeks to find a statement and now has three days. That is why a standing list matters more than it sounds: it lets the request leave on the third business day, before anyone has opened the books, based on what you know the client always owes.
This is also why the compliance items belong in the same list rather than in a reviewer note. Receipts that need substantiation, mileage logs and W-9s do not become urgent when you reach them in the ledger. They become urgent on a fixed calendar date set by somebody else, and a chase list is the only thing that respects that calendar.
Running both without confusing the client
Firms that use two tools should split them cleanly by message type. Documents the client owes on a schedule go out through the chase system on a fixed day. Questions arising from your review go out in one batched message per client per week. The failure mode is not the second subscription, it is three separate messages landing on a busy owner on a Tuesday, after which none of them get answered.
Agree the split before you roll anything out, tell the client which message means what, and keep the reply channel identical for both. If you cannot hold that discipline across a 40 client book, pick one system and accept the gap rather than teaching your clients to ignore you.