New packages for the "core" repo, for example the Linux kernel, are first introduced to the "testing" repo, which isn't enabled by default. You would have to manually change your pacman.conf to use it.
From the wiki [2]:
"After a kernel in core broke many user systems, the "core signoff policy" was introduced. Since then, all package updates for core need to go through a testing repository first, and only after multiple signoffs from other developers are they allowed to move. Over time, it was noticed that various core packages had low usage, and user signoffs or even lack of bug reports became informally accepted as criteria to accept such packages."
[0]: https://wiki.archlinux.org/index.php/Pacman#Skip_package_fro... [1]: https://wiki.archlinux.org/index.php/Pacman#Partial_upgrades... [2]: https://wiki.archlinux.org/index.php/Official_repositories#H...
As long as you use pacman properly [1][2] and go through your .pacnew files to make sure there's no breaking configuration changes you should be well off, but of course your millage may vary. Any major changes will usually have a news post on the homepage.
[1] One of the major sources of breakages related to improper pacman usage is using -Sy (Update package lists) without updating your system -Su (Combined into -Syu to do both at the same time) before installing something else which make break dependencies.
Basically what happens is you install something that wants a newer version of a library (which other packages which haven't yet been updated to the current versions which are compiled against the newer library) and your new install pulls the new library in with it without updating everything else that depends on said library. Generally -Syu (Update package list and upgrade system) is recommended to be done in all cases where you want to update your package lists.
You build packages from source when you install something from the AUR[0] or you can build from the Arch Build System [1].
[0] https://wiki.archlinux.org/index.php/Arch_User_Repository