Someone like LarryJ can no doubt answer more accurately, but it might be helpful distinguish between structural failure and controllability, and b) (as noted) kinds of turbulence.

Aircraft are significantly over-engineered in terms of the amount of g-force that it would take to cause a structural failure of a critical component (like a wing coming off ) There are indeed instances of CAT turbulence causing g-forces that are large enough to damage an airframe and when such g-forces are encountered the aircraft has to have a mandatory major inspection. I am not recalling, though, any CAT incident with a large aircraft where CAT induced structural failure that caused a crash (but this being FT, I look forward to being corrected )

Crashes due to turbulence-related controllability issues are another matter. Low-altitude windshear on short final is one of the most deadly (witness the DL (?) crash many years ago at DFW). Windshear is defined as a sudden and significant change in direction or velocity of the wind relative to the aircraft's heading. When you are on short final, low and slow, and, say, the wind direction changes suddenly from a 20 knot headwind to a 20 knot tailwind, the aircraft is suddenly trying to fly at speed relative to the surrounding air that is slower than the aircraft's stall speed. The results can be unpleasant; if the aircraft is low enough to the ground there is not enough time for the aircraft engines to spool up to give enough thrust in time to counteract the loss of airspeed. This is why, since the DFW crash, a much better system of assessing and reporting windshear has been developed and why, when significant windshear is reported by other pilots or by the automated systems off the end of a runway an aircraft's pilot will carry extra airspeed.

A similar controllability issue exists for what's called wake turbulence. When an aircraft leaves the runway on takeoff/the wings begin producing lift very intense vortices (the intensity depending on the weight of the aircraft, to oversimplify) come off of the aircraft's wingtips. Any aircraft following, particularly one that is much lighter, can hit one of those invisible vortices as it falls toward the ground and can have a very rapid upset and get "spun" in the same direction as the vortices. This is why there are specific procedures for takeoff for pilots following such likely vortex inducing aircraft, including the requirement for greater time between a "heavy" aircraft taking off and a lighter one following it.

Mountain-wave/rotor turbulence is a third kind of turbulence that can make for controllability issues particularly in light aircraft. Very intense "spinning" vortices can develop over mountain ranges and no aircraft wants to get caught in one of those, particularly light aircraft.

And just a note on terminology: what you or I might consider "extreme" turbulence is often actually only mild or moderate turbulence per FAA definitions. (That doesn't mean it's enjoyable ) A professional pilot can go an entire career without encountering severe or extreme turbulence (these would mandate an extreme post-even inspection, too).