The first-order classification is whether or not the package has a lid. In general, lidless devices offer better thermal performance due to the fact they are not adding additional junction-to-case thermal resistance in the form of a TIM1 inside the package and the lid itself. TIM1 is the material applied by Xilinx within the lidded device package to thermally couple the die to the device lid. Lidless devices allow the thermal solution to get as close as possible to the source of the heat, which is the die. On the other hand, lidded devices can be easier to design, as they are the more traditional approach to IC packaging, and off-the-shelf thermal solutions can be easier to find.
From a reliability standpoint, lidded devices do offer more protection to the die while lidless devices with stiffener ring offer higher rigidity. This allows it to have higher board level reliability (BLR) characteristics. BLR refers to factors that can impact the device adhesion and signal contact to the board such as shock, drop, and vibration. It also can encompass factors like board bend, flex, and deflection that can result in partial or full separation of the device from the board.
There is also a common misconception that lidded devices offer protection from contaminants entering the package body and touching the die, inner package substrate, or packaging capacitors while lidless does not. While it is undeniable that lidded devices do offer more protection from this occurrence, the fact that lidded parts have vent holes to allow off gassing and moisture evaporation means that lidded packages cannot be considered sealed bodies, and similar protections against such contamination can occur. While there are certainly differences at the macro level for the reliability of lidded versus lidless, overall both are highly reliable package types with similar considerations for designing thermal systems.