I've experienced 3 fails on 3 different modern production (1980's or earlier), prominent name-brand revolvers:
1. A single-action that would not cock due to a single grain of sand. It had NOT been dropped on the ground, but somehow got a grain of sand in the mechanism while being stowed in a vehicle for months. It required complete dis-assembly & cleaning to get it working again.
2. Another single-action would not cock because the cylinder release would not actuate. It had a lifetime warranty so I sent it to the manufacturer where it was discovered to have a broken part in the mechanism. Due to this kind of failure from a relatively lightly used revolver, it now enjoys a permanent residency in storage... no longer to be trusted for defensive carry.
3. A double-action revolver started having intermittent light strikes on primers AFTER I had adjusted the trigger/mechanism tension to lighten the trigger pull weight. Even though this malfunction could be classified as operator error, it's still good to keep in mind that even though the adjustment can be turned/loosened all the way, it may not be designed to function that way.
Although pistols can be more ammo sensitive... In my experience, I've had more TOTAL FAILURES from revolvers than from *pistols... *that is any pistol that wasn't already a total failure right out of the box (Tec DC9
anyone?)
All that being said, a S&W 640 is an awesome little revolver that I came inches away from buying myself.