If you have an example of how transparency makes the shadow more complex, please give it. I’m fairly confident there are no cases like that. Transparency in the background image or border doesn’t change the shape of the box the shadow applies to, and things like opacity affects the shadow too. The example you had in your first comment is also wrong, as shadows don’t apply to other shadows.

There might still be cases where a particular optimization doesn’t work and the browser falls back to a buffer.