As I understand it, the worst of the worst was the dragon 800 when it first came out. Polaris made several changes and some progress, but it was never great without going to a long rod setup and getting/keeping the clutch balanced and in good working order; also, cranking up the oil pump is borderline must to get them to live. The Pro 800 is basically the same motor with some minor changes. 2011s had the most issues: injector problems, too little oil, weak crank, and other things as well. They did switch manufacturers for the crank in '13 (went back to Fuji), which made a big difference, but there were always some inherent design issues. That said, as long as you do the top end as soon as compression starts to drop off, keep the clutch up to snuff, and have the crank and bearings redone at the first signs of trouble (crank runout off), they can go a long time. I've heard of even '11s going well past 5000 miles (with a couple top end refreshes along the way). Oh yeah, the Pro RMKs also have very marginal cooling, and that also contributes to failures. My daily driver is a '12, and it's been great, but it's got a long rod motor, TRS's remote thermostat upgrade, and added cooler.
As for what to look for, just walk away from one that's obviously been run hard and put away wet. Maybe the first place to look is the clutch. If you're seeing a well-worn belt, tons of side clearance, and lots of slop and play in the rollers and bushings and all, you can probably stop there. Next would be to do a leakdown check. 2-3% is pretty typical on all but the tightest top end, but 5% or more is trouble. Finally, pop the clutch and check the runout (takes a dial indicator; you'll also want the service manual for how to check, and you'd want the FSM anyway). I can't remember the spec, but that's the sign of a crank on its way out. I can't say the failures are all a bunch of fluff, but it's not that bad if you know what to look for and keep on top of the maintenance.
