Split lock washers always break into two pieces eventually. They tend to be brittle, so that can happen before install and at any time after that. Once that happens the joint has failed. The position of the author below is that if the mechanism by which the lock washer is purported to work becomes active then that the joint has already failed and the lock washer is useless. He defines a failed bolt joint as one that is no longer at the installed torque. Bolted joints are surprisingly complicated critters, there's a lot going on in one that isn't obvious.
If you really want to deep dive into why to not use them, as well as a HUGE amount of other similar knowledge buy a copy of
"Engineer to Win" by Carrol Smith. I've used that book as a reference for college Engineering classes because the class text books didn't do as good of a job at explaining the topics in plain English. Another worthy title of Mr. Smith's is "Nuts, Bolts, Fasteners, & Plumbing."
I met the man at SEMA a couple of years before he passed away. When reading his books you might get the idea that he was quite a character, and that's understating it! He wrote a series of books titled "___ to Win", but "Engineer to Win" nearly covers everything on it's own. What it doesn't cover that is in the other books may not apply to off-pavement use.
"Nuts, Bolts, etc." goes more in depth on those topics in it's title. He told me that he wanted to call that book "Screw to Win" in keeping with his other titles, but the publisher would not go for that idea.
If you really don't want a bolt or nut to move look into how to install Safety Wire. The bolt or nut can still move, but its a whole lot harder to do.
I do use internal and external star type washers. Not for their possible locking function, but for their ability to get thru paint and coatings to form a good electrical ground.
