IMHO, if using ZHA then there should not be any reason to migrate from ZBDongle-P to ZBDongle-E or vice versa, they should be equally good as each other. As mentioned the only reason is if absolutely want to use a Multi-PAN RCP with Zigbee and Thread on a single dongle, but I personally recommend against that as it is much better to use a separate dedicated radio for each wireless protocol (just make sure that not using the same channel frequency on two different radios).
Try to follow all these tips first → Zigbee networks: how to guide for avoiding interference and optimize for getting better range + coverage