NI looks to be an easier route for private imports. Once the car was registered new in NI or has been registered in NI before 31/12/2020 and stayed there, no VAT/import duty considerations.
Imports from GB are more complicated for private importers due to the VAT/import duty (although there are exceptions to both if you can find a VAT Qualifying car and using the EU returned goods relief scheme).
Dealers are an exception now as they have access to the VAT refund scheme recently introduced by the UK government but that is beyond the private importer (currently at least - or having to setup as a dealer to avail of it).