What functionality do Rocket Carts have when compared to the Onager? Cheaper to make, so more easy to make? Cheaper upgrade cost, like Heavy Rocket Cart. Do they have lower DPS, but extended blast radius and range? Faster fire rate or reload? Do they do pierce damage instead of melee so they are weaker against rams? Or perhaps a hidden bonus damage vs rams to off set that.
Do they break trees? Which is important for choke point maps like black forest/michi, amazon tunnel.
We don’t know yet.
Aside from the upgraded version being able to destroy trees they could have pretty much any stats. Maybe they even have pierce damage and no bonus damage against Rams so they are not a counter to it.
It uses gunpowder so I expect it to be quite pricy. Also the very fact that it replaces the mangonel that is very expensive to start with.
Nothing can be said until we see the unit’s stats and how it’s working, but given how it works I expect it to deal its damage over a wider area (poor accuracy) and by salvo instead of all at once. Better against large groups of units (more overall damage), possibly better range, but less effective vs an isolated target.
It’s just a light frame so low defence. So even more of a glass cannon than the mangonel.
I have it on good authority that after winning a battle they can fire a celebratory salvo of fireworks into the sky while your troops dance and celebrate their victory. 
I hope the rocket cart does something similar to this concet i posted some time ago:
Would be a great chance to implement a tweak like this.