First, ATACMS is a ballistic missile, not a cruise missile. You can't "shoot down" a ballistic missile like you can a cruise missile; you might, however, try to intercept it. But intercepting a ballistic missile is very, very difficult. This difference is why ATACMS are so important and considered a game changer.

So yes, most ATACMS missiles will find their target, and very few will be intercepted. Russia can't reliably intercept ballistic missiles in any meaningful way.

Second, ATACMS warheads, like the M39 Block I, weigh well over half a ton (590 kg). That's a lot of weight to toss 103 miles (165 km), but it's plenty big enough, especially when you consider that these are precision weapons, not weapons of mass destruction. These things aren't the weapons that make a big boom in the hope that they are just close enough to do damage. These are the kinds of weapons that will land right on top of the target and definitely destroy it. This is also why 240 is quite a good number. You don't need to carpet-bomb a field with artillery to hit one group of tanks and some infantry if you can obliterate the group with a single rocket containing 950 M74 bomblets.

Third, HIMARS (which is what launches ATACMS) stands for High Mobility Artillery Rocket System. The first two words of "High Mobility" mean that these launchers are designed to work in a fire-and-scoot attack mode. They can rapidly deploy to secure areas near the front lines and launch a salvo in less than a minute. Once fired, the HIMARS can depart immediately at up to 50 miles per hour. On a full gas tank, the HIMARS can travel about 300 miles (480 km). It takes less than 10 minutes to reload. At a maximum distance, without refueling, a HIMARS loading base could be positioned up to 150 miles (240 km) from multiple forward firing positions, and a single HIMARS could still complete four launches every day. This means the opponent does not know where the HIMARS will be firing and does not have time to return counter-battery fire after launch and before the target departs. This is especially true with artillery, which the Russians would be likely and able to use. The only primary alternative would be glide bombs, and those would be mostly trying to hit a moving target. (Which they would be next to useless for.) They might be lucky with a drone attack here and there if one happens to be in the area, or they could expend some missiles to target these systems, but that's unlikely to succeed because, unlike the Russians, the West, including Ukraine, does have systems that are proven to intercept ballistic missiles very reliably. And Russia has been mostly interested in hitting civilian infrastructure with its missile systems.

This is why everyone keeps saying that HIMARS with ATACMS is a game changer. Once you understand the system's capabilities and the Russian military's limitations, it becomes pretty clear the damage this combined system can do, particularly to back-field logistics and formations, and how little Russia can do to counter it.