Just a personal opinion to be taken with a grain of salt, but...

I HATE the trailers. Hate, hate, hate, hate, and hate!
They are heavier, more drag, wider, longer, slower...

My son (my experience) rode very well in the back seat kid carrier up until he was almost four. The weight distribution was even. He rarely moved around, loved the riding, he could look around and see.

Most importantly, there was a large visibility issue. Living where I live, a large percentage of drivers are driving HUGE trucks that have hoods/grills that obscure the trailers no matter how visible those flags are. We have had several instances where people have run over the cargo trailers (fortunately no children have been harmed). I never fell; my son and I rode daily to the park 3 miles away. He loved it so much that if he woke up before me I would find him with his helmet on, strapped into the rider waiting to ride off to the park. (and yes, he could buckle, but not unbuckle, so many time I woke to 'Mom, Mom, Mom, bike ride! bike ride! bike ride!'). Fortunately I had an bike rack in our tiny apartment that secured the bike upright.

Again, everyone has their own experiences, but I do NOT like the trailers.