I've not read a single negative review of the original run of 76's (i.e. Smith & Wesson production from 1967-1972~) in terms of reliability. They just run - I've had just one malfunction in 3,000 or so rounds of various brands/grain/quality. You will however find a fair number of negative reviews of the S&W76 in regards to their ergonomics, parts availability and military usage. Some of these conclusions are justified, others are not, and some of it is entirely baseless internet-regurgitated hearsay without any first-hand experience to give it credibility.
My opinion is that the original S&W76's are well built, incredibility smooth (especially when suppressed) submachineguns that offer a cool bit of military history and americana at an attractive pricepoint while representing the last example of open-bolt submachinegun development. Parts availability does indeed suck, however it's getting better with some aftermarket modifications although I look forward to straight replacement parts like bolts and FCG's. Another criticism is lack of versatility - my opinion here is that if you are buying a submachinegun for the purposes of popping an optic and VFG on it then you may be missing the attributes of the S&W76 and it may not be right for you (I have a Lage'd M11/9, there's nothing wrong with versaility, but it definitely isn't the selling point of the 76.)
In terms of S&W 76 vs. MK760 vs. Stemple vs. SW76/Burgess guns it comes down to two questions really - I) What can you afford?; and, II) What do you want out of the platform?. If you can swing the money ($10,000ish in the present market) and find the original form of the 76 appealing then by all means the S&W76 is the right call. If you need to spend less and/or want some versatility then look closer at the Stemple and SW76/Burgess guns. I'm half looking for a cheap Stemple because I don't mind some tinkering, like the cheap prices they can be had for and I would love a .45 version of my S&W 76.
