In my experience, major reasons of PTCRB stumble with pre-approved module is related to RF/antenna design (radio frequency).
- spurious emission by device
- interference with surrounding radio devices
- TRP/TIS (Total radiated power/Total isotropic sensitivity) as required by operators like AT&T
To reduce the cost and time, one should use PTCRB pre-approved module and get engaged with RF design house in early stages of development process.