Except that the longest nails a roofing nailer will shoot are 1 3/4". Not enough penetration for a piece of 1x4 or any other kind of 1X. Galvanized roofing nails are "washed" with different zinc and cadmium agents to keep them from rusting until used; they are NOT meant for completely exposed use. This is especially easy to see when you pull open a leaky roof and find all the nails really rusted. So much for the galvanizing.
It is not uncommon for me to have an open box in the back of the truck over the weekend and find them starting to have a thin dusting of rust on Monday that formed over the weekend. Imagine those nails where you can see them weather. Ever see one of those nice fences that has the long trails down the face side that start at the nail heads? Rust.
Shoot your fence together with a utility gun that shoots 2" galvanized nails that are made for Hardie, fences or other "exposure" applications. You can rent that gun for $35 bucks a day at the Borg and with the same gun I can put up 300' of 1x6 pickets on a fence once the posts are set and cut and the rails are on in a day.
Get everything ready to go (gates cut or built, material dispersed) and you won't believe how fast you will move once you get your strings set to line the tops.
You can do a >lot< of things with a utility nailer, but not too much more than nail foil insulation/hang windows/shoot off the gussets on site built trusses and of course, roof, with a roofing nailer.
Ahhh.... the quandry. If you have the roofing nailer longer the rent will eat you up. But if you buy it, there is less utility value down the road. But if you rent the utility nailer, you probably only need it for a day but you will lose the gun for all kinds of other neat uses if you didn't buy one.
Solution: Buy them both. Woohoo!
Whichever one you use the least sell on a Saturday night EBAY madness sale.
Robert