I can confirm that the amount of upgrade related issues has dropped to almost nothing at this point, though obviously they do come up from time to time. The one thing I can recommend on this is as follows, and ensures you'll never be server-down for more than a little while during upgrades:
1. Take a full backup of the SmarterMail install directory and domain storage locations before beginning your upgrade process.
2. Plan your upgrades for weekends/holidays if at all possible to provide yourself plenty of time to uninstall, install the previous version, and roll in your backup data to get things back online; if something goes wrong.
3. Set up a backup MX (can use free SM license) to handle incoming email while your primary is offline for upgrades. If the upgrade takes 5 minutes, or 5 hours, you'll be covered!
Technical Support Specialist