I have been using TDF since they started and IME it is a very small percentage of checks that don't get cashed eventually. (Although I don't use the very small denominations because I would assume more of those don't get cashed and those are only prepaid)
the info is there but admittedly hard to find
Oh, that is most certainly incorrect. I probed every part of the site, found more than 1k in checks over 3 months showing as unpaid some of it dating more than a year ago. Then after contacting them they updated the system and it showed $150 in checks unpaid. When I asked about a check that updated to "paid", they said they see it unpaid on their end. Another time a check showed unpaid, and shul kept sending an invoice. After calling up CS, found out it was actually paid. And if they ever credited back canceled/unpaid checks, I have yet to see an entry for it.
It's basically a mess, WYSI
NWYG.