-CakePHP is atrociously slow, even when running with APC, and uses an alarming amount of memory for what you get. Symfony2 is neither slow nor particularly memory-hungry.

-CakePHP is inexpressive; it's very difficult to use it piecemeal. You either use the whole whomp of it or you don't use it at all. Symfony2 is modular and easily used for exactly what you need.

-CakePHP is stuck in the past; any framework that is intentionally made compatible with versions earlier than PHP 5.3 is short-changing its users by not taking advantage of exactly what the language is capable of doing.

It has no real redeeming features. (Note that I wouldn't recommend PHP in any greenfield development today, but if you have to do it, Symfony2, or at least the Silex core, are no-brainers.)