My experience with cmake, though dated, is that it's simpler because it simply cannot do what autotools can do.
It really smelled of "oh I can do this better", and you rewrite it, and as part of rewriting it you realise oh, this is why the previous solution was complicated. It's because the problem is actually more complex than I though.
And then of course there's the problem where you need to install on an old release. But the thing you want to install requires a newer cmake (autotools doesn't have this problem because it's self contained). But this is an old system that you cannot upgrade, because the vendor support contract for what the server runs would be invalidated. So now you're down a rabbit hole of trying to get a new version of cmake to build on an unsupported system. Sigh. It's less work to just try to construct `gcc` commands yourself, even for a medium sized project. Either way, this is now your whole day, or whole week.
If only the project had used autotools.
No, CMake can do everything Autotools does, but a hell of a lot simpler and without checking for a gazillion flags and files that you don't actually need to but you're checking them anyway because you copied the script from a someone who copied the script from... all the way back to the 90s when C compilers actually existed that didn't have stdint.h or whatever.
CMake is easy to upgrade. There are binary downloads. You can even install it with pip (although recently the Python people in their usual wisdom have broken that).
CMake can't do everything autotools does, but the stuff autotools does which CMake doesn't isn't relevant anymore in today's world.
The fundamental curse of build systems is that they are inherently complex beasts that hardly anybody has to work with full-time, and so hardly anybody learns them to the necessary level of detail.
The only way out of this is to simplify the problem space. Sometimes for real (by reducing the number of operating systems and CPU architectures that are relevant -- e.g. CMake vs. Autotools) and sometimes by somewhat artificially restricting yourself to a specific niche (e.g. Cargo).