Wow...I would think the gun would have to be rare. I would not worry about the frame mounted de-cocker's reliability. Most of the problems that S&W encountered with this set-up was based on the imfamous FBI 1076 model. The problems were a result of specific bureau specifications that required a special hammer with a special trigger; Smith advised that they could have one or the other, but not both as it would cause problems. Apparently, the FBI spec'ed out the gun the way that they demanded it....and low and behold they had problems. These problems were later corrected on a case-by-case basis at the Performance Center. No such problems have erupted since then, although it is a rather rare variant. Unfortunately, the entire debacle left a bad taste in everybody's mouth, the guns didn't sell well, and were eventually yanked from the line.
I personally own a 1026 with a frame mounted de-cocker and have had no problems with it. In fact, I think that it is rather cool as I am a big Sig-Sauer fan.
If the price is right and you're in the market for a hi-cap midget 9, I'd jump on it. You'll probably be the only one on your block to have one.
Good luck.