The catch tends to be if you're able to detect when you're in a more complicated situation and bail out accordingly. One very useful thing is auto-labeling, which turns out, getting it wrong is more problematic than one would expect.

Take for example the 'Gorilla' issue that Google encountered.

http://nyti.ms/1JlTBab [www.nytimes.com]

Then there was the issue with the bookmarks and suicide categories

http://bit.ly/1JFkmCr [productforums.google.com]

(This one isn't as big of a problem, as the frustration was probably more due to the compulsory usage of the new bookmarks, but it still shows some hazards of auto-categorization).

With social things, it can sometimes be very hard to solve a problem that works for 95% of the population, but doesn't piss off the remaining 5% (often with good justification). Unfortunately, detecting when you're in that situation in the first place is sometimes equivalent to the problem you're trying to solve.