In a controlled feed rifle design, a cartridge slips under the extractor as soon as it is released from the magazine. This means than if the bolt is retracted before being locked into battery, it will pull the cartridge back out as it retracts. On a push feed action, the extractor does not capture the cartridge rim until the bolt is locked into battery. Thus if a push feed action is retracted early, it will leave the cartridge mostly in the chamber, potentially setting up a double feed malfunction.
On the other hand, push feed actions generally use plunger extractors, which are able to eject an empty case after a shorter travel, where a controlled feed must be opened completely to eject. This makes a controlled feed action more prone to unintentional short-stroking.
In practice, neither of these issues is really a significant concern and the hype over the difference is meaningless for the vast majority of applications.