Polyisocyanurate Board has one of the best R-values for sheathing but has several drawbacks you should know about:
1. As you already know it is not a structural grade sheathing and additional bracing is required if it is used.
2. It releases toxic fumes if it is ignited or exposed to flame. (Because of this it may not pass your local building code.)
3. It will deteriorate if exposed to UV radiation or sunlight.
If your local building code allows it‚Äôs use, and you decide to use it, I would suggest you use the foil faced. When installing it be sure to tape all the joints to prevent possible air infiltration. Diagonal wind bracing must be installed at all corners of the house.