It's not really right to say any of the traditional construction techniques cause failure. All the traditional construction techniques when used conservatively can produce frames that last a lifetime. If there was a construction technique that caused failure, then it wouldn't be used. Dead soft 4130 is still stronger than hi-ten. Frame failures are rare in general, and frames usually get retired for reasons other than spontaneous failure. Especially with fillet brazing, skill is going to be second guessed whenever there's a failure, and fillet brazed frames aren't as common as lugged or welded frames. Some construction techniques are just likely to be more prone to failure. Every single surviving traditional technique is adequate when used in a conservative and traditional manner. The issue is when people want to do things because they think it's better or deviate from traditional and time tested designs. If you want an authority figure to put your faith in about fillet size, that would be Keith Bontrager.