These are neat too.
2 downsides I can see, based on my limited experience:
- heavy loads will directly affect the bike's balance, since there's no torsional "give" at the hinge in the "roll" axis;
- related to that - if you lay the bike down, you could spill the trailer contents, since it has to lay down with the bike. So you'd either need a kick stand, or something to lean the bike against when you park
Small points, but there you go.