On Cryptomatte being like wire color, well cryptomatte is cryptomatte and is its own thing :) Advantage to other masking options is better handling of depth of field and motion blur.
Cool on Open As! Did not know that.
I do indeed just use windows rename to change the extension (then a lazy double click will open it in Photoshop through Exr-IO in the future hehe). Just to check, you have the Exr-IO plugin installed? Could be PS without Exr-IO says error due to the cryptomatte data, as I know it can't handle that on its own without the plugin ;) Could be it makes it think it's an invalid EXR since it has data and info it doesn't understand.