The big thing with sweat fittings are the pinhole leaks. They can occur from excessive flux, or from the fact that sweat fittings are just copper pipe stretched into a 90 or 45 or whatever. That means the material is not only weaker but exposed to more wear and tear from the water, as it has to more actively direct it. In cases with high water velocity, that means that the backside of 90s can be eaten away much quicker than the rest of the pipe. It's most common on the fitting immediately after pump.