Yeah, this isn't true at all. There are many busts of course, but there are equally as many times the top signings are very good. Top free agent in 2009 was Matt Holliday, who has been great for St Louis. 2010 was Cliff Lee, who worked out great for Philly for most of the contract, Adrian Beltre and Victor Martinez also worked out well that FA class. Prince Fielder and Pujols are bad deals long term from 2011, no doubt there, but have been a mixed bag so far (and everyone knew those were terrible signings at the time they were made). Grienke in 2012 was a fantastic signing for the Dodgers. Cano has been a bust from 2013.
If anything, recent history seems to say to stay away from hitting FA and stick with top end pitching FA signings. Outside of Sabathia (and with his weight issues that was always doomed to be a poor signing long term), most of the top end pitchings signings have worked out very well.