For the magnet to actually uncouple the cars, there needs to be slack in the couplers. If you're pulling a train over the magnet, the couplers are in tension, so there's no slack, no uncoupling. If you're pushing a train over the magnet, there is slack, but uncoupling won't occur unless you stop the train with a pair of couplers over the magnet. This is all under ideal conditions. If the loco on the train momentarily falters due to poor electrical contact, false, or unwanted, uncoupling can occur. To see how this occurs, set up your magnet with a piece of flex track and a few ore cars. You don't even need a loco: just use your hand to push or pull the cars over the magnet. Try a smooth push and a smooth pull over the magnet, then repeat with a hesitation or two thrown in, to see what happens. Also practise deliberate uncouplings, both pushing and pulling. This should give you some idea of the capabilities and limitations of magnetic uncoupling, and allow you to make your decision based on your results.
Wayne