I use Airflow Spey line and really like it. Most cracking and line fracturing is caused by the line striking rocks, trees and the ground on the back cast. Also, most fisherman overload the line with heavy streamers, triple nymph rigs with corkies etc. The line can overload and fracture during the forward cast, when there is no more stretch left in it. These are a few of the more common causes of line problems that most manufacturers see. They usually just take the hit and give the customer a new line, as the last thing a fisherman wants to hear, is that they caused the damage to their line.
If you need to cast big weight, it is important to use a balanced system, consisting of the appropriate rod wt. , line weight, leader and tippet sizes that the fly requires. Many rod tips fracture as a result of overload as well. I have snapped a few tips this year on my single handers, trying to throw 70' casts, with 10' of T-14 and tanden #4 Cone head buggers. This will usually result in a fractured rod tip and/or a couple of tanden #4 Cone head buggers to the back of the head.
The manufacturing processes used to produce todays modern line systems, are very accurate and consistent. Quality control measures have virtually eliminated bad lines reaching the market place.