I own 3 S&W revolvers with the lock on them. The lock hasn't been a problem on any of them, MIM parts haven't been a problem. Two have been back to the factory, the third should go back but it shoots ok so I haven't sent it back. The problems with the guns have been quality control related in all 3 cases. Neither gun that was sent back was repaired to my satisfaction but both now work.
If you want a model that was made in the time frame where they pinned the barrel and recessed cylinders on the magnums, buy one of the older guns, there are plenty of them around. If you want a 625JM or a 686 plus, you can expect to return it for warranty repair and when you get it back it still may not be right. I recently bought a 60's model 15 and couldn't be happier with it. I also have a 70's vintage model 19, 60's model 28, they're all great but the new guns kind of suck out of the box.