I get the impression that the original idea for the Nexus was that it was to be a place that everyone wanted to be, that you could live out your fantasy (much like a holodeck). Then they had to explain why Kirk was still alive after 80 years, then they thought lets make the Nexus stop aging, then they had to show Picard what would happen if he let Soran go, then they made the Nexus allow you to travel back in time and then had to explain how Picard jumped around in the Nexus and then they allowed everyone to move around space and time. So the Nexus became sort of 'magical'.
I would have forgiven all that (there's been worse in Star Trek) if they didn't make the Nexus fantasies for Kirk and Picard so lame . And they hadn't gone against Picard's and Kirk's established characters to say Kirk wanted to cook eggs for Antonia and Picard the perfect father.
At the beginning of the movie Picard is grieving for his family, and his mind is full of "what ifs." Kirk apparently always wondered, if subconsciously, what life would be like if he hadn't left Antonia. The Nexus gave both of them a look at what could have been.
As for jumping around time and space, I always looked at the energy ribbon as a portal into a non-linear dimension. That's what allowed them to leave wherever and whenever they chose.
It's really no more of a suspension of disbelief than anything else Trek requires.