Great for the developer. The user doesn’t use Mac, Windows and Linux. Just one for work and one at home, with mostly different apps, so they couldn’t care less if it looks the same on different platforms.
They may care, however, if they get anything at all. I do not have the resources to target something to all platforms, so the alternative wouldn't be "Users get UI targeted towards their OS", the alternative would be "Users get nothing since developers don't have the time to also target their system".