Haven't read all the responses, but here's my take on things.
1. Every well-designed character should have more than one role. If they are hyperspecialized in one thing, then they're going to be bored a lot of the time. Plus, assuming a group of 3-6 players, there are a lot of roles that need filling, anyways.
2. Countersnipers. Basically, the other side has snipers, too. Take care lest the hunter become the hunted.
3. Unaware only lasts for the first shot. Once people know they're under fire, the situation changes (its just returning fire that gets difficult until they spot where it is coming from).
4. The other side isn't (always) stupid. If they know there's a sniper on your team, they'll set up meets indoors, for instance, or in public spaces where a sniper attack will draw all kinds of hell down upon the team.
5. Snipers are virtually useless inside office buildings.
6. Every well-designed character should have more than one weapon skill if combat is any part of their role. Yes, they may have a 6(+2) for Longarms (Sniper Rifles), but it would be really good if they had a 2(+2) in, say, Blades (Swords) or something. Yes, I am fully aware that a katana is not very concealable. However, especially in some parts of the sprawl, an openly carried blade is far more acceptable than a sniper rifle, even in 'decent' company. Same with a pistol or staff.