autoconf is nix-only in practice, macro-based, verbose and has a significant legacy burden. I have suffered plenty of pain from trying to build the few projects that still use it. In such cases it's often been faster to write own build scripts for some other newer buildsystem.

CMake and Meson I've used across tens of different projects, there's solid online documentation, significant usage, a good underlying language and I already have my toolchain files ready. Using autoconf instead of something more common and modern is likely to turn off people contributing or using your project as they'll be unfamiliar with it and are going to have more trouble solving issues on their own.