This is not about cmake vs. autotools. This is about C++ ABI problems between compilers. Notably MinGW/GCC and MSVC.
The Problem is that in the past GpgMEpp had a CMake build system. This allowed it to be compiled with MSVC and linked against a MinGW compiled GPGME from a binary dev package.
Since switching to autotools we no longer have the option.
This hinders adoption for people who require (for whatever reason) to combine GpgME++ and QGpgME with other Code compiled with MSVC. This was possible in the past and no longer is possible.
I would accept having a parallel Buildsystem for the C++ Language Bindings (Libraries QGpgME and GPGME++). And for CMake I would be willing to maintain this in parallel.