Your definition of durable exceeds its normal values. Firstly you use it equally as tough, wich scotts are not, but secondly you ignore the facts that scotts have higher HP than any other mobile light arty. As for it takes 3 shots it means you have plenty of time to retreat, since the normal ambush uses 2 AT guns at most. Lastly because scotts have such a high performance in other role than their intrnded role (indirect fire cover) OP asked for a discussion. Not its durability, not his mobility, but its AI autoattack power. Nice derail there tho.
But it's never been used nor it's main intended role is to be used as an indirect weapon as mortars or artillery. The unit is closer to a mobile light armored Brummbear. At most you'll see someone use the smoke from time to time.
It's ingenuous to compare it to rocket artillery, because they behave in a different way (long cooldown, no AA, fire most of it payload in 1 go). That's why they have an HP pool of 160. Because they don't need to be in the frontlines to work.
The closest thing is the MHT and it's still quite different even if we don't compare it's cost, popcap or time or arrival. Let's take a look at garrison clearing.
Mortars have a 0.50 modifier against garrison on direct hits and have a far dmg of 20 (this is used to calculate the dmg they deal to all entities inside a building despite whether or not they are close to the impact of a shell).
The Scott is set to 0.25 and 10dmg respectively.
Thinking a bit more, the closest related unit to the Scott should be the Stug-E.