The problem with little upfront planning is the team might spend too much time on certain features in the beginning and get crunches at the end to deliver an MV

The problem with little upfront planning is the team might spend too much time on certain features in the beginning and get crunches at the end to deliver an MVP experience at the agreed upon deadline. By signaling early that a deadline may be missed, there is an opportunity to make course corrections in the spec or shift folks from other projects to deliver a higher quality experience at the agreed upon deadline.