I've upgraded Debian stable (both pure and with some cherry-picked backports) and Ubuntu (non-LTS and LTS) systems in place and rarely broken anything, for years and years. When stuff has broken it's been a quick google and then slapping myself for not having read the upgrade guide.
I do generally wait about 2-3 weeks before upgrading, giving time for them to catch stuff that was missed until the great masses were set loose on it.