There are three main issues that need to be over come.
1. Under normal circumstances you need to be within 30 feet of your target to attack with a ranged sneak attack.
2. Once your first attack goes off, your opponents are considered "aware" of you and so additional attacks will not receive sneak attack damage.
3. In order to stay hidden, you will need to spend a move action to hide again, which means you only get a single standard action to attack even if #2 wasn't an issue.
The Range Limit (30 feet for the Un!Rogue), lack of a stat-to-damage (Un!Rogue's Finesse training only works for melee weapons), lack of feat/weapon support, and lack of full-attack options make this really tough to pull off. Plus, you take a pretty painful penalty to Stealth checks.
That said, I could see a Dagger-wielding switch-hitter working decently. I might also see an Elven Longbow Rogue working okay, but it would be really MAD.
Certainly there are challenges, but they never struck me as being insurmountable. The sniper feats, a sniping weapon and goggles of sniping eliminate most of the issues.
Oh, Nice ninja!
Yeah, Sneak Attacking at range is tough but it can be done. Another build that our party Sap Master abuses: Light Crossbows with Training Arrows provide a decent ranged option until they can get to melee range.