would hold up but if the boards are flexing or moving the poly will crack, it has no flex.
You stated that the poly is cracking where the boards are meeting. Try sanding down, and using the poly again, being careful to only paint each individual board only and not making a bridge of poly from one board to another, ie in and over the crack or bond.
Get a poly with UV protectant and apply several light coats instead of few heavy coats. This stuff is great, if not interested, consider a marine protectant. This will also do great for this application.