Most of the movie is clearly shot in front of a green screen.
It makes no sense for the future of Hollywood Bld. to be a forest, without any residual traces of having once been a city. That would make sense if Robert was to go back in time, approximately 1,000,000 years or so. That would have been before it's eventual developed into it's present day thriving city. Going forward however, he would either find the city now in ruins, or even more advanced.
There has been some speculation from those who have viewed this movie that the old man is actually Robert. In that case, a time paradox exists in which there are two individuals who are in fact the same person.
What should be the central plot of the movie is entirely dropped after Robert meets She-Ba. It stops being about time travel and becomes a sci-fi.
Several things regarding She-Ba and Robert's first encounter with the saucer. First, they should not have been able to gain access. Secondly, being alien technology that they have never before encountered, they should not know how to wither fly the saucer or use it's weapons.