For full trailers, the law in many US states reads somewhere around less than 2/3 of the towing vehicles weight, without proper brakes. This for automobiles , US laws make almost no mention of even the existence of bicycle trailers.
Lots of experience pulling bike trailers and the 50% rule works pretty well. Axle mounted always seems better than seatpost, all things being equal.
I've pulled many bicycle trailers:
Wike Moonlite, 40lb kid plus 20 lbs of his junk, pulled very well. About 85lb total, I was 175lbs. Stable and solid, even at 45mph.
Bob Yak, rated at 70 lb capacity, barely stable at that weight, try 30 lbs.
Burley Nomad, will haul an 80lb toolbox, making gross trailer weight 95lbs, hard to stop but pulled and tracked o.k. stable at 45mph with 45 lb load.
S. Winchester Originals, steel child trailer, tracked well, but a little heavy. Hauled about 40lbs reliably(no kids).
Burley Travoy, seatpost mount, folds well, it feels wobbly and handling and braking go to hell starting at 35lbs cargo. Empty it's fairly tippy, so you don't want to drop it off of a curb. Pulls beautifully empty, I don't take this one over 20mph.