I have an 18U rack sat ontop of these, about half full on a hardwood floor.
It rolls away, not because the floor is uneven, but because whenever I go to push a jack into a socket the unit is pushed away from me... then it keeps going...
Credit where credit is due, the bearings in these wheels are really good... maybe too good. Thoman should invest in some breaks on these wheels and you should consider if you really need your rack to move, and how you plan to stop it moving when you don't want it to... I use a doorstopper.