Glasgow Central no longer has the facilities for vehicles to enter the station area, and the nearest road with space to do that (Gordon Street Infront of the station) will eventually be made pedestrian only. It's also a good 100m from some of the higher numbered platforms.
Additionally, about half of the Central Station throat closes overnight for maintenance on weekend nights (checking the status of the great many double slips). The station is however in use overnight most week nights from 3am to 6am with the sleeper and avanti, and so is theoretically open for use during this time.
The problem comes with late arrivals and departures. If it's scheduled to use Central overnight, but arrives late, there isn't any flexibility. The peak hour there ramps up quite quickly, with absolutely no free platforms during the day.
Overall I'd say probably beneficial from a financial case point of view, but NR aren't going to want an as yet untested project in the middle of the busiest station in Scotland overnight unless these problems can be overcome. I'd argue Elderslie, Mossend or Grangemouth still remain the most likely.