We have sent wheels to the Olympics... the hubs don't slip and those are high quality steel washers which are not made of pot metal and the bolts are high tensile M10 that uses an 8mm head.

If you want beefier washers those are not hard to find, we just keep them separate from the allen bolt so they can be replaced.

My own Arvon wheels have some nice SA lock washers which have never budged under some very hard use.