Yeah, i don’t like BiDir either, in fact I like more hybrid techniques that dedicate an specific part of the power towards collecting some specific paths, that’s how light tracing works in LuxCore, and I suspect that it’s also how Progressive Caustics works in Vray.
In fact Light Tracing is not executed in the GPU in LuxCore, it’s always executed in the CPU, but it’s also a very fast technique since it’s only calculating those paths, so usually with a 5% or 10% of the CPU dedeicated to that you get super fast caustics, however the current LuxCore technique don’t support SDS and it needs Caustics Photon Cache to support SDS, but both of them together make a very powerful caustics engine, and it runs in parallel to the PathTracer (the photon caustics cache works in the GPU too BTW).
Now sure how progressive caustics in Vray works in this regard, may be a more advanced version of the light tracing algo that also supports SDS, I don’t know.
But doing hybrid techniques I think is the way to go to get everything together.