I had a master link break on my old 2001 Bandit back in 2002 or 2003. When it failed, the chain was thrown and broke a chunk off the clutch pushrod (?).
A year or so ago the clip came off my dirt bike's master link while I was on a ride about 20 miles from home, and the link disengaged and the chain went shooting out the back. I had some spare clips but as shitty luck would have it I launched them off into the woods as I was trying to install them. I ended up just putting the master link on with no clip and riding home in second gear at 15 mph (to minimize any excessive chain jerk). Incredibly, I made it. As I arrived home and killed the engine, I heard a "clink" noise, and the side plate for the master link fell off.
In the case of the Bandit, I had a backyard mechanic do the work and I actually spec'd out a non O ring, extremely cheap chain, in an effort to save money. In the case of the dirt bike, I had bought it used and had really never gone over the chain and master link; if I had, I may have noticed that the clip was not correctly installed, or whatever.
Despite those anomalies I continue to run master link chains and trust them. I have reached a point in my motorcycling career where I do my own chains and so forth so now any screw ups rest on me... both my Stroms currently wear DID X ring chains with clip style links, and in a cumulative 90,000 miles neither has had an issue with that setup.