Courtesy of ChipandCo, there are some reports that there are body and physical requirements that guests need to know about for both Flight of Passage and Na'Vi River journey.
Bigger Guests
Apparently bigger guests (heavier, taller, etc) said that they could not meet the restrictions.
"The ride vehicle is a “link chair” and looks similar to the middle part of a motorcycle with a seat you will straddle. There are restraints that secure your back and legs and will need to “lock” in place before the ride can begin. Many guests are reporting no issues but some have been turned away because of the seat not being able to fully engage. Currently there is no modified seating option for larger guests who may need a little extra room."
So once again, very similar to Seven Dwarves Mine Train, Disney has catered to smaller body types and made another ride too small for bigger people. I am 6'4" and a former D1 football player and I could not fit on Seven Dwarves. Some NFL players friends of mine also could not fit on the ride and could not go on with their kids because it is geared for smaller people.
I'm not sure what the recent game plan is, but lately Disney seems to be going for smaller seating or smaller cars and as a result ignore specific body types.
in addition, Disney does not have demo link chairs to sit in outside of the ride to see if you can fit before waiting 2-3 hours to get on the ride only to find out you cannot ride it.
Here is the load up instruction video for Flight of Passage:
Apparently Disney did issue a statement that they plan to install a demo chair at a later date.
Disabled Guests
If you have a disability you will currently have to move from your wheelchair to go on the Na'Vi River journey. They have no wheelchair accessible boats at this time. This seems to also be an issue with Flight of Passage because you have to straddle a link chair in order to experience the ride.
With all of the time Disney had to plan out this park, perhaps they should have focused on these issues during the process.