EDIT 2014-11-05: It's fine to
fedup to F21 now - at least so far as I know, and so far as the bug described in this post is concerned. We've made sure several different ways that you should not possibly be able to hit the 15 minute timeout bug.
It's probably not a good idea to try and upgrade to Fedora 21 with fedup right now.
Currently Fedora 21 has a build of systemd that includes a new feature that was added upstream after the release of 216, which is intended to time out system startup if it's not complete after 15 minutes - the idea being to avoid things like your laptop melting / starting a fire in your bag if it gets accidentally powered on, stuff like that.
Unfortunately, turns out that having a timeout that hard powers down the system if boot hasn't completed after 15 minutes doesn't work very well with fedup, because while fedup's actual 'install the updated packages' step is running, systemd considers that boot has not 'completed'. So if you try and fedup to Fedora 21 using a fedup environment that has the affected systemd build (like the one in the Beta tree, and also in the current 21 'stable' tree), and your 'install updated packages' boot takes more than 15 minutes, it'll just suddenly cut off and shut down. Obviously, there's quite a high chance that'll leave the system in a broken state.
So: don't do it. Really, don't.
We're currently investigating the best way to deal with this problem, and we'll certainly try to have it all straightened out by Beta release date (Tuesday). But of course, it's never a good idea to upgrade a production system to a pre-release, especially if you don't have good backups!