Personally I see Meson as an attempt to do CMake right. A big differences between Meson and SCons is that SCons handles the execution of the build graph, while Meson delegates execution to Ninja. The nice thing about the former is that while the graph is being walked, new nodes can be added. The nice thing about the latter is that it's incredibly efficient.
Why not both? :-) While the architecture is more similar to CMake, Meson's language reminds me of SCons---but it is a custom DSL and not Python, which makes things less surprising. For example in SCons a single-element list can be replaced with with the element itself, but that is a feature of SCons's builtins: if you write custom Python code, you can have confusing results when you place a string in a place where you must use a list of strings.
Also SCons is Turing complete and this makes it a bit of a stretch to call the language declarative; Meson keeps simple loops but is not Turing complete and the maintainers are okay with plans/pull requests that make Meson even more declarative. In Meson generally "there is only one way to do it", in SCons much less so which is ironic given it uses Python as the language.
All these systems annoy me with their dependencies... the only one that i kinda do not mind much is premake because it consists of a single self-contained executable. However even with that you need to have premake available. I'd prefer it if these programs took a page out of Autotools and created configuration scripts that only relied on whatever the target OS has available (plus the compiler) without having the need to install a separate build system since after a while you end up with a bunch of different build systems since everyone prefers theirs.
Meson's only dependency is Python, with no external packages needed---only the standard library.
The only annoying part of Meson is that it only supports a few compilers (GCC, clang, ICC, MSVC), and requires porting to new compilers unlike Autoconf.
