Incandescent dimmers are designed for "resistive" loads, and typically rated for at least 600 watts. They have a requirement called "minimum holding current", which is the minimum amount of electrical current flowing through them to keep the dimmer firing properly.
Cree LED Lighting downlight and lamp products have electronic drivers that are "reactive" (capacitive and/or inductive) and draw very little power (10-11 watts). These two factors result in electrical current that is just barely above the minimum holding current for most dimmers, and below it for others. There are other factors specific to the installation site, such as neutral-to-ground integrity, power line noise coming from the utility or other loads, and source impedance. With the minimum holding current requirement barely being met, these other factors can cause the current to dip below that threshold, resulting in occasional dimmer mis-firing which can result in a faint flickering coming from the lights.
Cree LED Lighting tests its products with a variety of dimmers, and makes recommendations regarding compatibility based on those results, but cannot guarantee performance.
The LR4™ and LR6™ product families are designed to dim down to ~25% of full brightness, then turn off when dimmed below that. Due to the nature of LED light, this can appear more like 40%-45% when looking directly at the fixture. Examining the room being illuminated will give a more accurate perception of actual brightness.
The CR6™ and LR24™ products are designed to dim down to 5% of full brightness. Due to the repetitive peak currents generated by the inrush current of LED lighing fixtures, Cree products should be treated as a 75 watt load when sizing dimmers.