This was my problem with taking the motorcycle taxis when I lived overseas. They were completely unmarked and really just guys with their motorcycles looking to make a few extra bucks. And it was like yeah, it just doesn't seem safe to ask some guy I don't know to take me somewhere and then hop on the back of his motorbike and let him drive me away in a city I don't know. At least the car taxis were usually marked.
Marked or not, I think it just comes down to common safety and discernment- but overall most taxis or ubers are safe.