Follow up question: when using ranged weapons, how much care was taken to avoid friendly fire in large skirmishes by the Romans?

I'm curious, which source advances the claim of 60+ scorpiones? That seems like... a pretty extreme number?

Also, engines were big and required transport. Now if you were operating in the Greek East with lots of fortified cities then you'd likely bring along your full complement of siege equipment and if you encountered an army - well I have no idea but I don't really ever recall of their usage in open battle.

However if you are operating in say Gaul where the defences were less robust then it's unlikely you need to bring the full complement. We do know that some were brought along in Caesar's campaign as he has anecdotes of their effect.

The important thing is that even 60 scorpiones, even if mounted on a small two-wheeled cart, is going to take up physical space in your army train. A train that is already hundreds if not thousands of wagons in addition to the thousands of mules allocated to the individual legions. Each additional wagon or cart is two or more animals that will require fodder and increase your logistical problems. Each lengthens your supply train which in turn reduces the your march length.

60 carts carrying ballistae pulled by 2 donkeys will require 600kg of fodder supplemented by grazing each day (and 2,400L of water). If you plan on marching for a week that balloons to 4.2t of fodder. So now you need more wagons to carry that extra food for the siege train - who in turn need to be fed meaning you need more wagons... etc. It also increases your problems because you need to find grazing land for even more animals. I think we begin to see why an army probably wouldn't bring along all those engines, despite their potential efficiency, if it meant they'd be running out of food before they could reach the next major settlement.