I started working with CUDA at version 3 (so maybe around 2010?) and it was definitely more than rough around the edges at that time. Nah, honestly, it was a nightmare - I discovered bugs and deviations from the documented behavior on a daily basis. That kept up for a few releases, although I'll mention that NVIDIA was/is really motivated to push CUDA for general purpose computing and thus the support was top notch - still was in no way pleasant to work with.

That being said, our previous implementation was using OpenGL and did in fact produce computational results as a byproduct of rendering noise on a lab screen, so there's that.