I guess the most unique thing about most optos is that the optical cell saturates based on how intense the light is and for how long. So that creates a natural variable release that is usual characterstic of opto compressors. If you hit it hard with a chord, the cell will saturate and take longer to release. If it's just being hit lightly, or very short transients, it will release quickly.

And some of them have a two stage release. They'll release about halfway pretty quickly, like tens of millis, then the rest of the release is the variable length release, which can go up to a couple seconds if you really saturate it.

Those two things really make for a particular type of compression style that, as long as the part isn't too fast, is almost like a very good human riding a fader as you play. Vocals of course are the biggie that they do so well on, other than perhaps rap, because vocals tend to have the kind of shape that it rides perfectly.
You can emulate that with other circuitry imposed on non-opto type compressors, and various compressors do just that. But the optos do it naturally.