I agree aircraft are the most reliable way to stop low flying cruise missiles however not the only way. The s-300 and s-400 use l band linked through multiple sattelites as well as many other radar systems, I have no doubt they can see a cruise missile but I doubt they can actually target them until they are close, as many of those radar systems have limitations especially towards low flying targets, meaning they could likely see them coming with l band but have to wait until they are in their face to strike. However there is also the pantsir system with accompanies every s-400 system and is also used by syria even though it has no s-400, the pantsir was designed from the ground up to handle artillery, low flying cruise missiles, and attack helicopters exploiting terrain for quick strikes, and pantsir is not a new concept it is just a modernization of an old soviet system designed to do the same.
OK, first of all the PANTSIR is unlike any system the US uses, but is closest to what we would have if we combined the AVENGER with a PHANLAX CIWS system into one platform. It is a very small missile, combined with conventional type guns.
Yes, this type of system can do a much better job, but it still has a great number of limitations. But it is still far better than any of the more conventional S-200/300 type of platforms. To begin with, it is designed to launch it's missiles with as little as 10 degrees of elevation. This is sharply different than the others, which launch at closer to 90 degrees of elevation.
But the power of the RADAR or the band has nothing to do with the simple fact that RADAR does not perform well (if at all) at low elevation targets. Ground clutter, backscatter, even obstructions (trees, buildings, mountains, changes in elevation, etc) make these platforms extremely "near-sighted" at low elevation.
In attack type RADAR terms, we use the phrase "Visual Search Lower Boundaries". Quite literally, we go into the software and block out huge areas because things like this confuse the systems, as well as other things which can effect performance. Trucks, flocks of birds, dust, all of these at low elevations tend to show up as "positive hits". To such a degree that they make it impossible for the crews to function. So the search software blocks anything out in these areas.
The only real exception is if they are able to put the RADAR up high, on top of a building or mountain. That for example is why non-mobile RADAR sites are pretty much always elevated. By putting the RADAR on top of a mountain, you pretty much eliminate this issue.
The conventional missiles can not hit low elevation targets because they simply can not get to them. A missile like PATRIOT or S-300 is designed to hit distant and higher flying targets. They take off at a sharp angle at a high rate of speed. Generally they can not even begin to maneuver until they are between 500m and 1km of distance from the launcher or more. And at that point they have that much elevation or more.
Oh, and they are also facing away from their target. Something most missile systems are not designed to deal with. In fact, most systems can not be fired at a target they can not "see" themselves. So it does not matter if the RADAR sees it, the missile can not so will refuse to launch.
PANTSIR is more akin to AVENGER than any other system that has been mentioned. A CIWS system could get them also, but only at point-blank range, as a final defense.