https://streamable.com/ytp674
Watch "The moment a Storm Shadow missile strike on Sevastopol" on Streamable.
The Storm Shadow have very similar capabilities to ATACMs, I don't think it will massively increase what Ukraine is already capable of doing.
I think the reason the US is dragging their feet to release them is that once Ukraine starts using them, Russia (and their allies, Iran, China etc.) will have enough data to start developing countermeasures against them.
We are seeing a very clever move away from batch delivery of weapons, toward a sustained Ukrainian production model that will be less vulnerable to allied political shifts. I notice that while the ATACMS is not headed to Ukraine yet, another large money infusion is. I wouldn't be surprised if the US is pushing hard to assist in spinning up Ukrainian production of their own Hrimm 2 missile, which has similar capabilities. While it is in service it doesn't seem to have a good production pipeline yet. If the Neptune land attack cruise missile variant is an indicator though, it may only be a matter of time before a larger and larger share of the weapons being used to encourage the occupiers to leave will be built in Ukraine. Either under license, or their own designs.
In fact Storm Shadow has a much more sophisticated warhead and using it outside of its specialised role (penetrating a building like seen here (even much more fortified ones), then detonating deep inside) because you don't have much with that range is an actual waste. But that's where ATACMs fit in perfectly.
ATACMs is more a generalized and weaker missile with a few warhead variants (mostly bomblets or unitary, fragmenting on explosion) mostly used against at best medium armored targets while doing a lot of area damage. But then it can be fired more easily from all the existing platforms using GLMRS.
Storm Shadow on the other hand is a completely difference beast. A warhead twice the size and specialised to breach very hard targets including buried bunkers, where you can even pre-program how deep it penetrates before the main charge goes off. So when you really want to hit what's deep inside a building quite precisely, this is the right tool.
Oh, they simply are cool... up to the created backwards-acronym that brought us B.R.O.A.C.H., the warhead designed to breach fortified constructions and bunkers (pre-charge to clear soil covers above a bunker, more than a ton of mass at high speed to smash through the top and additional levels and a main charge that can be programmed by timer or intended depth... You want an explosion on floor 3, of 5-floored bunker? Here's your tool...).
Damn, look at those invaders just hanging out like they're not in a warzone on temporarily stolen land. Boggles the mind.
Go home invaders. Go home.
An unconfirmed rumor that these missiles killed Russia's Black Sea Fleet commander Admiral Viktor Sokolov