The M14 program was pretty horribly mismanaged, to be fair to the "bean counters." A ton of R&D money and lots of time was spent to ultimately field a product-improved Garand that was basically no major improvement over prototypes that existed in 1945. In the finest tradition of really bad government procurement debacles, the M14 also had a couple good lies wrapped up in its development, namely the claim that it would be cheap to make because existing Garand tooling would be usable and, of course, the claim that 7.62x51 was suitable for an assault rifle.
All that said, the real flaw with the M14 for general service rifle use was the cartridge more than the platform. Just about everyone on the planet except for the imbeciles in charge of US Army Ordnance had actually studied combat from both World Wars and drawn valid conclusions about optimizing infantry rifles for how combat really happened. Army Ordnance refused to learn anything from their own data (or anyone else's) and instead forced an overpowered, bulky and heavy round onto NATO when what was needed was something like 7.62x39, 7.92x33 or 7x43.
Rommel said that in a close range fight (and most of them are) the victor is often the guy with one more round in his magazine. We'll never know how many guys died along the way with M14s, FALs or G3s in their hands because the other guy with the AK had ten more rounds in the gun and a larger basic load in his kit. Or because he could get accurate enough shots off faster with his AK.
But, an M14 in, say, 7x43/280 British would have been a different animal -- still not a real big step forward in terms of state of the art in terms of ergonomics and such, but it likely would have lasted longer in service when it ran up against the AK-47 than the M14 as issued did.