- Waste bandwidth - both speed and amount of data
- Reduce your computer's performance without warning
- May force you to restart, and not give you a choice
- For large updates like service packs, it's best to let people beta-test it first, and at least backup before you install them.
- May reduce functionality or introduce undesired features (ads)
- Give software unfettered access to your computer (spyware and rootkits)
- Have caused massive system outages in the past (McAfee 5958 in 2010)
Google Software Update on OS X runs in the background, cannot be disabled easily without hindering Google software, and gives you no option to turn it off! You know, the same company once said "don't be evil" too!