"If frictionless trade is good for NI then why not for the rest of the UK ;-)"
Because it comes with a sting in the tail, that is, loss of sovereignty for that part of the UK. NI may prefer that to having a hard border with the south of the island, but it's that, and the concern that having a hard border might trigger off the murderous scum, that makes them a special case. But this surely just points out the obvious, sorry about that.