Erlang/Elixir and kdb+/q full-time developer here.
I didn't had difficulty learning kdb+/q. But that's maybe because I had an early exposure to APL (I read a chapter on it in a book as a kid).
Also I had a previous work experience in HPC, data-parallel computations and vectorization/SIMD using the array-programming languages such as Matlab, Octave, R, Julia, etc. And GPU Compute using CUDA/OpenCL.
The really hard to learn part of the kdb is a proprietary frameworks around it. You need them in order to develop a production-grade systems.