Warrant canaries are unreliable. Apple, for example, stopped issuing warrant canaries in Sept. 2014 [1], but was added to the NSA's PRISM program almost 2 years earlier, in Oct. 2012 [2].

But I suppose a warrant canary by nature requires companies to be honest. There's not much you can do if the company is being deceptive with a vaguely phrased canary.

[1]: https://arstechnica.com/tech-policy/2014/09/no-apple-probabl...

[2]: https://arstechnica.com/tech-policy/2013/06/new-leak-feds-ca...