So looks like I won't be posting comparison images of our materials, because at least with Corona 6/3ds max, the caustics appear to be "broken." The image can never clear, I tried doing region renders on small regions to be sure.
The photons being shot into the scene for caustics are large, there aren't enough of them, and there is no or not enough interpolation between them, causing large bright spots (and I don't mean fireflies, those could be removed with denoising) to be present in the scene and never clear. The denoisers are useless for this, because they only clear small "pixel size" noise, not large bright caustic photon splotches (see attached image for a better description of this effect in general, this is not what I am seeing specifically, just an example that illustrates what bad photon handling looks like, although in the sample image shown it was done intentionally).
I tried toggling adaptive caustics mode, even tried playing with the developer/experimental caustics settings, including maxing out the number of photons (per pass?) to 99,000 and playing with _many_ permutations of the other settings - well as many as time would allow, since rendering with caustics on is as slow as watching Pluto rotate around the sun, and I did want results some time this century, especially given that the hardware I am using, although not a 64/128 core threadripper, is far faster than what 99.99% of people are using (Skylake X 18/36 cores at a fixed 4.9 GHz with -1 AVX2 (also fixed) and custom loop direct [cold plate to actual silicon die - no IHS] water cooling to keep temps ≪90 ℃ while using AVX2 full blast, like Corona does). Nothing helped, the caustics mode is completely useless for this scene and Corona does not allow enough control over photon settings to fix it, not to mention that having to twiddle with these is reminiscent of trying to get caustics to work in VRay/RedShift/Corona Physical, etc..., with the million knobs you have to turn to get caustics right (or to appear at all in the scene!).
We love Corona for its simplicity when it comes to render settings - its greatest selling point! But, as this scene clearly shows that the caustics simply do not work for a scene like this. I don't know if this is a bug, a deficiency in the specific algorithm that the Corona team chose to use for caustics, or what.
If someone from the Corona team wants the 3ds Max Sample scene that demos caustics completely failing due to very poor caustic photon handling/calculations, I can post it as an attachment. It is just a variant of the scene already posted in this thread by pokoy with some tweaks.
Or is this a known issue with caustics - that they just don't work for some scenes with partially transparent objects? I mean the scene is not overly complex, by any means and the lighting is trivial, with two area lights.