There is no perfect way to hitch a trailer to a bicycle. The perfect hitch would attach to the contact patch of the rear tire, or maybe to the ground ahead of that. Clearly impossible. With a multi-bar linkage, you could create a virtual pivot at that point, and simulate such a hitch but it would be complex, heavy, and introduce a lot of slop. Not at all practical.

All the practical hitching methods have trade offs. Most of them can be made to work, and work well enough that the user won't notice any downsides. So I am sure to annoy someone who has a setup that works "just fine" by pointing out the downsides of that setup. Hopefully, being aware of the downsides will help trailer builders/buyers negotiate the tradeoffs and end up with something that works well for them.

The steering pivot is the hitch, basically, but some trailers like the BOB have separate steering and pitch pivots, and two wheel trailers need a roll pivot as well, so the bike can bank and the trailer keep both wheels on the ground.

For good directional stability, that steering pivot needs to be at or ahead of the contact patch of the rear tire, or the axle. This is the main issue I have with most of the commercial one wheeled trailers. They put that steering axis behind the rear tire. This can still work if you keep the trailer light and the speed down. Loaded heavy, though, this really wants to fishtail in any situation where the trailer is pushing on the bike..stopping or braking on a hill.



With a two wheeled trailer, seatpost hitching can create a problem due to roll-steer interaction. As the bike leans back and forth (for example climbing off the saddle) it also steers the trailer right and left. Depending on lots of factors, this can lead to instability. In general it will be stable when the bike is pulling the trailer. The pull of the trailer will be trying to stand the bike back upright. When braking, though, the trailer will be trying to push the bike over even farther. With a light load in the trailer, the rider may accommodate this effect without even noticing it. A heavier load will require some effort, and a really heavy load may overwhelm the rider.

When a bike leans, it pivots about the point where the tires touch the ground. The higher up you go the farther that point moves side-to-side when the bike leans.

Hitching a two wheeled trailer to the chain stay or dropout greatly reduces this issue, because nearer the ground, leaning the bike doesn't steer the trailer nearly as much as seatpost hitching. Note that this is still not perfect, and you could still get in trouble with very heavy trailers. Also, this hitching method limits how tight you can turn, and may have heel clearance issues depending. Also, to avoid pulling the bike to one side, the trailer needs to track centered on the hitch point, not centered on the bike. Trying to fix this "problem" leads to a real problem of the trailer fighting the bike.

With a one wheeled trailer the hight of the hitch point doesn't matter. It leans with the bike no matter how high the attachment point. Attaching to the seatpost puts the steering pivot well forward of the axle, so directional stability is excellent. The trail-a-bikes all use this, and as long as the joints and attachment are not sloppy, it works well.

There is an issue with seatpost hitching that effects both one and two wheeled trailers: Wheelies on acceleration/hard climbing. When starting out, the mass of the trailer is pulling rearward on the seatpost, and especially on a tall, short wheelbase bike, it may be difficult to keep the front wheel on the ground. When stopping this will tend to reduce the weight on the rear wheel, making it easy to skid, which can cause loss of control. Many bicyclists do not make sufficient use of the front brake in normal circumstances, so may have difficulty stopping such a trailer if they are unwilling to learn to use the front brake.

ALL hitching methods will be more stable if you keep the load low and forward.If the load balances behind the trailer wheels (negative tongue weight) then stability will be poor in all cases. For stability, the mass of the trailer must be centered ahead of the axle... and the heavier the tongue, the more stable, but a too heavy tongue can cause other issues, so we are back to tradeoffs. The BOB puts the axle well back and the load very low to gain some of the stability it loses to too-far-aft hitch steering.