The Brexit advantages are the same as always. Out from control of the ECJ - out from the customs union and single market meaning we can do our own deals - and control of our borders. This has never changed. It's called taking back control.
Let us examine that. Maybe tomorrow (not literally, but sometime soon), UK makes a trade deal with the Klingon Empire (to avoid specifics of any real-world example). What will the relation between NI and Klingon Empire be?
Will NI be able to avail itself of the KE deal for exports? For imports? Or neither?
Will NI be able to make its own deals with the KE?
If NI cannot either make their own deals, nor avail themselves of UK deals with KE, as I suspect, what sort of bizarre country do we live in? Where the avowed intent of the UK locks a constituent of the UK into situation in which they cannot benefit? No matter that at some point in some possible future they can change their CU membership.
Does that mean all UK trade deals will have to expressly exclude NI? Does that mean that the KE could object to their UK deal being expanded to include NI?