The processor unit of a receiver must be extremely fast in order to arrive at a nav solution in a timely manner, particularly if the receiver has significant velocity. Processing may either be time-sliced between satellites, or some receivers are capable of concurrent processing via multiple, parallel channels. Objects like, say, for instance, mountains or mountain ranges. This limitation on our view of the sky is probably the single biggest problem in the backcountry. If you're navigating across an open expanse of the sea, without anything blocking your view of the sky, it's not a problem, but in the backcountry we've potentially got lots of topography.
Trees further limit the reception of the satellite signals.
And how often are we following trails which themselves follow drainages down in the bottom of gullies. Getting a simul-lock on at least four satellites in such a situation can be downright dismal. Another factor that needs consideration is cold temperatures, and their effect on batteries in a receiver. All that intensive number crunching can eat batteries, and cold temps can sometimes severely limit battery performance. Just turn it on when conditions favor a good view of the sky.