I am pretty sure that you can put a bionx on the cetma as well. I think Marla Streb has one on hers. I definitely wouldn't run down the bullit, but I definitely find the cetma to be more comfortable and, counterintuitively, faster than the bullit. I have never found any cargo bike particularly hard to ride.
The workcycles bike was what made me know that I needed a cargo bike. It is very accessible and easy to handle. I did not think that it was slow, but i did find that it discouraged me from putting too much effort into pedalling, in the end, it wasn't sporty enough for me.