1. Needlessly opaque
2. Prone to abuse from over zealous admins
3. Google does it wrong (Checking the header chain all the way back instead of the last system the recipient does not run)
4. Breaks email standards
5. Doesn't solve any issue that SPF does not solve more directly, without possible abuse, and much more simply, requires far fewer CPU resources and skill, and does not break email standards in the process.
I'm told that "I'm too stupid" to know how it works and "I should get out of computers since you obviously are too stupid to know your f'ing job!" (both quotes from right here on slash dot). I won't try to prove otherwise, but one question I've asked over and over again is how DKIM, checked back further than the last untrusted relay, does not break email standards for list or forwarded mail. SPF won't break those, DKIM will, every time.
So getting back to our muttons, I'm not surprised that Google's spam engine (or anyone's, for that matter) has a high false positive rate, or a lower than desired true positive rate. That issue is simple - they are attempting to solve a problem with technology that isn't technical in nature. Stop using a hammer to try to screw in a light bulb. Doesn't work well.