There are only two ways in which sustain can be affected by a pickup
- Its very close to the strings, so its cutting vibration of the string short
- Its a very hot pickup, so it can pick up less vibration from the string, and that can give the illusion that the string is vibrating for a longer time than if you had lower output pickups in (when really its just detecting lower amplitide vibrations).
Beyond that sustain is a matter of the guitars resonance - the pickup cant make the string vibrate more or less (unless its really close to the string in the latter case).
FWIW when I've used or played nailbombs (assuming its A5 youre talking about) I've never had a problem with sustain or pinches, and I have ceramic nailbombs in 2 guitars and the sustain and pinches I expect from the guitars are there.
If youre having a problem then I think youre doing the right thing checking the wiring. Dull, cloudy joins can attenuate the signal from the pickups and high frequencies especially, and the pots might also be cr@p and should probably be changed.