That can be mitigated with "barn doors" or some other form of shading device used to exclude the background from the sensors view. That's not always needed or beneficial. Many, if not most times, simply pointing the front of the unit at the subject is enough.
The other wild card is the camera in play. Reasonably modern TTL metered cameras (like a Nikon N90s or F4) with fairly modern lights (say an SB50 which lacks its own sensor) that can talk to the camera and get instructions from the camera on how to fill as the shutter button is pushed.