I was trying to use PHP_CodeSniffer 2.x standard on PHP_CodeSniffer 3.x binary. Since ruleset.xml format wasn't changed I wasn't getting any exception due wrongly named sniff classes.

I however did get this output:

ERROR: Referenced sniff "Squiz.PHP.DisallowObEndFlush" does not exist Run "phpcs --help" for usage information 

Unfortunately exit code from that operation was 0 (= success) and therefore any build system using PHP_CodeSniffer wouldn't fail.