That's bad at the best of times, and considering it broke multiplayer compatibility, I'd say that was unacceptable.
And everyone should have learned this with GSC when it came to Chernobyl. I'm not defending them. But all this and more are the reasons why I have not yet bought Clear Sky. At some point consumer logic has to take over and you need to recognize that you had warnings and made the purchase anyways. So why be shocked when the same problems with Chernobyl are occurring with Clear Sky?