Too much air, too many collisions
They pick up twice as much yes.. for the same amount to be collected, meaning it just takes more time. On paper they're better, in practice..
Fully upgraded runners, without Gorilla arms, have in my experience delivered more ore back home faster
The rocks do float in the air for a while, it would be nice if they had more gravity. Haven't seen too much collisions yet but i'm not very lategame. It doesn't take more time, they collect a pile, throw it to the maw, then collect another pile to throw it again or walk it back. That's a ton of time saved compared to walking there and back.