I think it would be a grave mistake for them to join. They would lose all bargaining power right away and they'd be joining a very dysfunctional EU.
They are already paying the price for not being de jure members. (Currency exchange costs, rate hedging is also costly, and they don't benefit from EU unified foreign policy - the current Trump scare for Greenland could come for them too - hence the argument to join.)
no doubt in my mind I'd vote no in their place