If you are working to ASME B31.3, have a look at the table immediately following Table D300 (the main SIF's listing). You will find that a weld neck flange has an SIF of 1.0, a slip-on flange (double welded) has an SIF of 1.2 and a threaded flange SIF is 2.3 for instance.
The signficance is of course, the relative time to failure under cyclic loads. In reality, a lap-joint or a threaded flange both have hot-spots, which will logically lead to fatigue failure faster than the smooth taper shape and butt weld in a weld neck flange.