It is possible to win with the codex. I just did versus Necrons........
Not to spoil the mood but beating the most outdated codex in the game isn't exactly hard to do. Sure there are some builds with Necrons that can be very competitive like destroyer spam but it comes short vs every 5E codex. Sisters I do not count as a 5E army though, as it's worse (my opinion) than the 3E one.
I will be closing this thread in a few hours so unless I can get some new rumours or anything it's time to say goodbye to this thread for now.