We do not endorse the use of any potting compound. They have the potential to affect inner component coupling, antenna match and other factors, especially for modules that do not have a lid.
That said, many customers do pot the modules with success. While we do not recommend any specific compounds, be sure to use one that is free of conductive materials or heavy in carbon. If the module does not have a plastic or metal lid, then it is a good idea to cover it and provide an air gap around the RF circuitry. Potting impacts not only the module, but also the antenna. Consideration should be given to the antenna traces and connections, especially if using a PCB or embedded antenna. There is a strong likelihood that the antenna will need to be re-tuned to accommodate the affects of the potting compound.
Regardless of the material, plenty of time should be allotted in the development schedule to test various materials and make sure that the potted radios still meet the product requirements.