Nice mate in 5
My first thought was like KingPan's: couldn't black just play b1=Q and have a queen for white to reckon with?
Then I realized, if 1...b1=Q, 2. Ra8+! Kb5 3. Rb8+! wins the queen and also prevents the other pawns from promoting. So it technically would take longer to mate in these lines, but mate would still be inevitable.