> PBRT's MediumInterface system can only represent a single "inside" medium and a single "outside" medium per shape. If a shape physically touches multiple different media (for example, a glass sphere sitting at the interface between water and air), PBRT cannot directly represent this configuration.

I think this is kind of odd for a renderer which is otherwise quite capable. Can anyone explain why this is the case, and how I can work around this limitation?

Straw man.

> Shadows have a hard edge, as only infinitesimally small point light sources of zero volume can be simulated

Uh, no. Raytracing can definitely have emitting surfaces and volumes.

> Reflection / Refraction can only simulate a limited set of light paths, for perfect mirror surfaces, or perfectly homogeneous transparent media.

You sure about that?

> More complex effects like depth of field are not supported.

https://www.povray.org/documentation/view/3.60/248/

Also, the title should get a "2019" tag.