I wrote a multi-target SIMD-using synthesizer, and I don't think I would've had as much success if I were just using someone else's library - its been especially important to have the SIMD instructions for both architectures I'm supporting (ARM and x86) directly available in the code, since a synthesizer necessarily involves a working pipeline from one stage to the other. I wonder how much easier/better the code would have been to write with portable_simd ..
https://github.com/seclorum/SIMDSynth
(uses SIMD for optimizing voice allocation and filter calculations on both ARM and x86, same codebase ..)