There is not a single company or tech that can garuantee you that if they mount a dish to your roof that it wont leak, that being said the best way to seal a roof mount is with using pitch pads applied to the bottom of the foot plate over the four outer lag holes and the top and bottom center lag holes. It is also important for the two center lags to anchor into a rafter for the greatest structural stability wich is critical if your in an area that gets alot of wind.
Pro's its up out of the reach of your kids, the nieghbors kids, your brothers kids, your kids friends, and if you have a line of site issue and the roof and or chimney ( please have the tech try for the chimney before the roof ) then thats your other pro.
Cons, there is a possibility that it could leak, this possibility goes up drasticly if the tech doesnt seal the mount location properly ( see my description above ), if the tech doesnt fully seat the lag bolts, the dish can move if the mount location doesnt not provide adequate stability such as its not into a rafter and or the wood is rotten, its up at a location that requires the tech to leave the ladder wich can be a problem in the winter if there is snow on the roof or if the roof has a steep pitch that puts the techs personal safety in danger, it can look unsightly.