I have them on my 5er - and annually pull and inspect them.
the rubber plug on them routinely breaks down and I have to replace it.
A couple of years ago, i had checked the plugs and they were ok, but when I arrive at my destination, two had broken and that meant dirt could have contaminated the bearings.
Off came the tires, repacked the bearings after a good cleaning to be sure no dirt got inside of them.
I like the idea, but in reality the old school checking and packing annually reduces the amount of excess grease the will expand with heat to a minimum. And, as others have said, it'll foul your drum brakes if it expands too much.