I've never used a roasting bag so I might be talking rubbish....
As I understand it the use of roasting bags is to contain moisture. Part of roasting potatoes is about driving out the moisture to get a fluffy result.
Your tatties may turn golden but their texture could well be affected.
Would you put foil over your tatties, which is not too dissimilar to modern roasting bags?
Neti they will go brown but not crisp, in fact they will be rather soggy. Same if you put foil over them, additionally with foil they may not brown too well.