If you give a winger a farrow you are saying to him that when you are in possession the formost end of the arrow is where his new position is (FL,FR). If you give him forward runs he will makes runs from either position so in effect what you are saying is right. He will run to the byline more often.
Where that COULD fall down is if you give him instructions to cross from deep which contradicts his instructions and therefore shouldbe avoided. If you give im forward runs he should cross from the byline only. At least that is my interpretation.
The downside of FWR and Farrows together might be if you are playing away perhaps. Because your winger will regularly be up the field as those instructions are very attacking oriented, there fore in a defensive tactic they would be counteractive.
Make sense? In other words I'd only use it in an attacking formation.
Hope that helps.