For power devices, the shoulder washers themselves matter a lot: likely being made of resin, more flexible (and weaker, so thicker too) than a metal washer, so they will compress much farther. At which point (at 10x full compression), the split washer is nothing more than a badly made flat washer. It's hardly anything! You probably stop cranking when the torque is over 10 times higher than it is while you're still cranking down on the washer.

Consider how much torque you apply while a split washer is being compressed. ^^^ ONLY conical spring (Belleville) and wave or ripple washers (the stiff ones, not the usual soft ones) have enough force and displacement to keep a joint under useful tension. Anyway, for total locking - that's what locking nuts, loctite, lock wire and split pins (cotter pins to the US folk) are for. I would use a toothed washer if any, it has at least some ability to bite against the surfaces (unless it's compressed flat though against a surface harder than the washer I imagine) and prevent rotation, which is after all what this is all about, if that's vibration or thermocycling, it's the bolt or nut rotating you are trying to stop. As you know, there is no shortage of official comment on split washers in that regard that from respected organisations (a 5 second google will give you references) a split spring washer 'seems' like a good idea on the face of it, but when you actually think about it, it's kinda obvious that it's not doing anything really very useful. If the bolt came looser from it's torqued position with the split 'lock' washer in place, then all that's going to happen is that the bolt will continue to work itself looser and looser until it falls out, the torqued joint has already failed by the time the compressed flat washer becomes a spring again.

Order of lock washer and flat washer