RF is your enemy. Both your GPS receiver and your datalink are sensitive to RF interference. RF interference will increase the time it takes for your GPS receiver to lock, increase the likelihood that you lose lock and reduce your GPS receiver’s position accuracy. Note that RTK (or carrier phase) GPS receivers are especially sensitive to RF interference. RF interference also reduces the range of your datalink.
You won’t notice any effect on data link performance when you are flying close to your ground station – where most flight testing occurs. When you are close, your datalink signal strength will be much stronger than any RF emitted by the systems on your drone; however, as you move further away from your ground station the datalink signal strength decreases rapidly with distance. The RF signal emitted by other systems on your drone does not decrease with distance from the GCS, and eventually, once you are far enough away from the GCS, the RF emitted by onboard devices blocks the RF received from your ground station.