Yes he's sitting on the edge. It's a 3" wide lip, comparable to the comfort of a bicycle seat. It would be very easy to attach a 2x8 to the lip for a more comfortable seat - there's a good 5 inches of flat just around the corner from the front lip and adding a bench wouldn't be an issue. At times I stand on the coupler for extra weight on the drive wheels - might be nice to add a standing platform to that, maybe attach through where the hitch pin is.
It works great on _dry_ hilly uneven land. I can add more articulation if I please, I would have to mill or grind a slot rather than use the clearance in the pin hole.
On a wet hill with a full load, traction is definitely an issue. I've had to get off and push. Wheel weights or engine weights would help.
It's actually very easy to steer with all that leverage on the handle bars too.
What direction are you worried about it tipping over? Sideways I think you'd have a hard time tipping it unless your load is very top heavy. Center of gravity is really low on both the BCS and the trailer. Tipping like BCS wheels off the ground shouldn't happen with the length of the coupler and the weight of the BCS.