It used to happen all the time, (70s-80s), even with some factory steel levers and the first aluminum ones. Usually never happens these days with any factory or good aftermarket steel or aluminum ones. I sometimes still buy Rocky Mountain's house brand parts, drive, levers, cables, etc but it's a crap shoot. Usually their brand stuff is ok, but it seems to be made in various countries/factories. I've had Primary Drive chains and sprockets last as long as RKs or DIDs, then bought the exact same parts and they last half as long. Rocky Mountain will almost always replace junk parts with no problem. It is usually half price initially, so sometimes I take a chance, and usually am satisfied, but not always.
I've never had a steel or aluminum MSR, Moose, or factory lever fail before it's time in the last 20 years. My theory is that for where I ride in rocks and stumps and roots a steel lever is better because if you smash it into something the lever will usually bend or break rather than snap a shaft. I've snapped shafts on several bikes with strong aluminum levers, even a couple times with safety cables attached, (usually called brake/lever snakes). I've also broken shift detent bolts off inside the case, the bolts that hold the shift forks in place, and that has only happened with aluminum shift levers on a KTM and twice on a Kawasaki. The strong shift lever transmits impact to the shift fork, and either the shift fork bends, the shaft breaks, or the shift fork bolt breaks. Once on a KX 500 the shift lever was bent and cracked, the shift fork bent, and the bolt inside the case was broken off, all from one impact.
If you bend a steel lever it will usually bend back and be useable. Even if it snaps, you can usually use pliers to put the bike in a gear and get out of the woods. I've even finished races with Vise Grips clamped on what was left of a lever and could still shift. In the old days we always carried extra shift levers just in case. The factory guys usually had complete motors, wheels, handlebars, levers, all laying out by the truck ready to replace to finish a race. Some guys would have a complete extra bike with all the bolts finger tight sitting on a stand ready to pull off parts for the race bike.
For those of you who didn't ride before about 1985, and especially in the 70s, it sucked! Motors blew, transmissions, parts, brakes failed, and suspension generally sucked, but at the time we didn't know any better.
I've used the tape trick around the shaft, Loctite, super glue, even aluminum foil. It's hard to find but there is a special Loctite that has about the same hold factor as standard red, but expands twice as much. NAPA can usually order it, but doesn't stock it. Air Force and Army helo mechanics can get it. If your shaft is good, and you use a good brand lever, properly tightened, with Loctite on the bolt, the Loctite on the shaft is just a precaution, but why not?