I know of no correlation between hand-built wheels and broken spokes. If there were, one cause could be the customer making unwise choices regarding rims, hubs, and/or spokes.

Another cause could be poor build quality in DIY wheels. In general broken spokes result from inadequate spoke tension such as on the rear NDS or large spoke-to-spoke tension variation either on one side or side-to-side. Proper wheel design and rigorous building standards can avoid this.

Another cause can be poor quality spokes. Some stainless steels and spoke manufacturing techniques simply yield have very poor fatigue failure resistance. If you stick with either DT or Sapim spokes, you will not have this problem.

Another possible cause of spoke failure is using spokes that are too heavy (too thick a gauge). I know that sounds backwards, but it is true. You need to stretch spokes significantly for them to resist going slack as the wheel is compressed at every point on the circumference on every revolution and especially when subject to impacts. Going slack is what causes spokes to fatigue at the ends. At any given tension a thicker spoke is stretched less than a thinner spoke. Spokes almost always break at one end or the other where fatigue occurs. Since almost all double butted spokes share the same gauges at their ends with heavier spokes (2.0 mm at each end), they are just as strong at those points as the heavier spokes. But they don't go slack as readily because they are stretched more. Therefore, the lighter spokes are more durable than the heavier spokes.

Finally, it is imperative that spokes be "stress relieved" during the building process. Both machine wheel builders and hand wheel builders (who are inexperienced) often omit this step or do it completely wrong. That causes a large portion of the broken spokes.

I hope all this helps.