It's not a copy.
Or at least a true copy.
Certain elements of the action were "borrowed," but it wasn't a true-faith copy job. S&W lifted what they believed to be public domain aspects of the design and then threw in their own touches.
Mechanical and material specs are likely QUITE a bit different.
Composition of materials, method of manufacture, care taken in manufacture, all can play a pivotal role in how durable/reliable a gun is.
One common problem I hear with the Sigmas, or at least the early Sigmas, was splitting of the polymer frame under the muzzle, leading to feed & function problems.
One thing I will say about the Sigma; the grip is one hell of a lot friendlier for my hands than the Glock is.