In short, the spice gifts prescience to all who use it in heroic quantities (Chani, for example sees Paul in her visions) but only the true Kwizatz Haderach, the product of a millennia of careful breeding, is capable of navigating (lol) the possible futures to use prescience to its fullest. The Guild, by comparison, are mere dabblers. Their limited prescience is a side-effect of them using the spice to avoid danger while using the Holtzman drive.
“I’ll pull their fangs presently,” Paul said. And he thought then about the Guild — the force that had specialized for so long that it had become a parasite, unable to exist independently of the life upon which it fed. They had never dared grasp the sword . . . and now they could not grasp it. They might have taken Arrakis when they realized the error of specializing on the melange awareness-spectrum narcotic for their navigators. They could have done this, lived their glorious day and died. Instead, they’d existed from moment to moment, hoping the seas in which they swam might produce a new host when the old one died.
The Guild navigators, gifted with limited prescience, had made the fatal decision: they’d chosen always the clear, safe course that leads ever downward into stagnation.
Dune
Note that even Paul, with his visions, isn't capable of seeing as far as his son (the God Emperor, Leto II) into the future, largely because his upbringing and moral compass limit him from exploring decisions that take him outside of his comfort zone.
As to the extent of their visions, the Navigators can only see to the next "nexus", a deflection point where their decision has ramifications on the production of spice in the future. As the book opens, we seen them running headlong toward just such a nexus. Paul's arrival is blinding them to the future, hence their willingness to allow House Harkonnen to attack House Atreides.
When the Arrakis Affair boiled up, the Spacing Guild made overtures to
the Bene Gesserit. The Guild hinted that its navigators, who use the
spice drug of Arrakis to produce the limited prescience necessary for
guiding spaceships through the void, were “bothered about the future”
or saw “problems on the horizon.” This could only mean they saw a
nexus, a meeting place of countless delicate decisions, beyond which
the path was hidden from the prescient eye.
Dune: Appendix III
