What Tigerbiten said. I had issues with the originally supplied old style hitch. They would constantly bend up and deform so as to make them non-functional. The newer hitch type seems to work better.
I have no idea how much I've used the trailer - a fair bit. I haven't had any problems with the wheels though. They seem as straight and true as the day I bought them - and that's despite getting the trailer caught in a pothole and flipping.
I do have another problem with it now, and that is not being able to take the wheels off. The axle has a push button which must move a slider that allows a captive ball bearing to drop back into the axle, allowing the wheel and axle to be removed from the frame. However, I'm guessing the grease has dried out and that mechanism has seized. Which means I'm in a spot of bother should the wheels need to come off the trailer for any reason.
I've done over 25k miles with mine, mostly loaded with camping gear while on tour.
I found out the hard way twice, the original bearings tended to die after around 5k-6k miles when fully loaded.
Then when changed to better quality, the spokes lasted around another 5k-6k miles.
But even running with only 2/3 of the full number of spokes, the wheels stayed true.
As for the axles.
If they are frozen enough so you can get the nuts off the end, the wheels comes off that way.
If the axle turns when you try to remove the nut, then it's penetrating oil and a hammer to get the axle out of the frame.
Done both methods to remove the wheels when having to replace a bearing.
Now my axles a glued into the frame, as it makes it a lot more predictable when trying to remove a wheel.