I've had 2 sets of SKS guards crack completely and 3 bridges where they attach to the brake mount snap as well. The plastic guards and flimsy bridge simply can't cope with road vibrations at high speeds.I had a Chromoplastic rear 'guard that vibrated itself into two pieces three times at about the 12 o'clock point, between seat stay bridge and rear 'guard stays. Fitted a Tortec in replacement and it's been fine.
I've seen other complaints on fora about them breaking, but as SKS are popular, it's hardly a statistically significant conclusion. As you say, the race blades are different.