CyanogenMod doesn't have any way to distinguish which phones are currently receiving security fixes in a timely manner and which are not; nor do they have any list of security advisories covering packages they distribute (go look at any notable desktop/server Linux distro — they all have public lists of security advisories and documentation of what release fixes them).
To my knowledge there is no Android distribution that has anywhere near the cohesive security story — and they're all miles behind any desktop OS.
And looking at my phone, I have no idea how I'm meant to update to the milestone builds. The updater lets me select "stable" or "all (inc. nightly)", and nowhere do the milestone builds appear…
None of this is helping me believe there's really any decent security story. Abandon all users who don't check the website (or whatever) to find out about releases, trusting the built-in updater to provide updates. Never publish any security advisories that cover your distribution…
Same. The newest version which supports my phone is years old, and there are major usability issues, particularly in the dialler interface. The manufacturer (HTC) UI was miles better (i.e., actually usable).
Maybe you need to install CM 11 first, so the system has a base to work with.
I think you can see if your phone gets updates if it's listed as CM11-supported.
I even asked the sales representative if it would be kept up to date (knowing I couldn't trust them, but was looking for any extra assurance), and they said yes.
Right now it's at 4.3 and wont upgrade...