For one thing Britain will be free to negotiate its own trade deals with others. For another. Britain regains it judicial sovereignty after a short transition period. The rest of the U.K. other than NI will not have to abide by EU standards.
Under the May deal, the UK would have been free to negotiate its own trade deals post Brexit.
The same for the judicial sovereignty - that’s the same in the May and Boris deals.
the only difference is that the customs union is only for NI, not for rUK