Not convinced on the shaken baby syndrome from a bike especially for a 2 year old minimum age, and I wouldn't let my child not be strapped in, he would wriggle free if he wanted to once something other than the ride distracted him.
I would be concerned about impact damage to his chin on landing a jump my kid wasn't prepared for, but I guess that is an age thing.
I use a floating type rear seat, which is fine for trails, if it is really rough I bungee strap it to my seatpost with locking carabinas which reduces the movement.
I bought it from halfords for £40 :shock: it is a rebranded Hamax. I do like the kangaroo seat but see no point in replacing the one I have as it has lasted 3 kids [7 years] and only has another 2 to go.