Interesting piece, I must say.
I wasn’t even aware of this term but now that you mentioned it, I realize much of Medium is surrounded by ghost buttons!
I do feel it makes sense here since content is king on this site. Also it works when they are clearly visible like the Medium top navigation to access your alerts and profile. It also works for actions users will be doing often like clapping and responding. You expect them to be there so no need to highlight them.
However, content is not always king on every page. Surely, a splash page is there to just grab your attention vs inundating you with information and so why hide a button? I can understand it may look better and if I do use a ghost button on a splash page, I’ll make sure either the entire page is clickable or at least a good middle portion of it is clickable. It would be a shame for the user to leave thinking its a one page site.
Another area where the button should not be ghost buttons if the page performs a very important action like buying an item on an ecommerce page. While you may argue content is king, content usually takes up a large portion of such a page with pictures and descriptions so a brightly colored or prominent button is only fair. In the end, all those pictures and descriptions are mere supporting players to get you to do one thing — buy the item. That’s the sole purpose of an e-commerce site!
I design a lot of web applications versus web sites and the former usually have a central purpose of helping the user at his job. There is no doubt in my mind, most users would hate ghost buttons for this reason. These buttons represent actions that they are trying to accomplish so they can do their jobs and so to hide them or not make them pop would be a terrible disservice especially when these buttons are often found in a sea of data and data grids.
Some of this seems pretty common sense because while ghost buttons make terrific sense in say the New York Times site so not to distract readers, you can bet their Subscribe button is not a ghost button. Maybe Medium should follow suit to get more subscribers.