My spouse and I have had our two FNARs since early '09, put a few hundred rounds through them, together, and never had anything like a FTFeed, FTEject or jam of any kind. Several different kinds of ammo, including lots of W German Mil Surp NATO 7.62mm mfr'd in 1969(!). Had them in the woods a bit, banged them around some, no concerns/issues. I just don't think you're going to have any problems. The piston-driven action is plenty "energetic", and there is a gas-feed adjustment valve into the action-drive cylinder accessable in the field with one flat-blade screwdriver (probably a dime would work). Kinda the consensus over at
http://www.fnforum.net/, FNAR area, is that the FNAR comes from the factory with this feed screw set pretty "open" (energetic cycling), and I think the rifle is heavy and smooth enough that this works fine. Others think you should get better POI repeatability shot-to-shot (do you need better than 1 MoA?) if you close the action valve down to the minumum needed for the load you're firing to cycle the action - then and only then AFAIK you get some FTEs.
Given reliability, the main things are: It works exactly like you'd expect and it is a joy to shoot. Some complain that the trigger is a little too heavy (rumor is 7 pounds - just got a Lyman gauge so I'll measure and post) for a bench/target rifle. BUT, it is NOT a bench rifle. It is a DMR meant to be used by police/LE scrambling around in emergencies, so, like an MBR or an assault rifle (but less so), you don't want light trigger pull, for safety reasons. But, if you do want that, it is known how to fix that too.