Ultimates should scale off the your highest stats, either magicka/spell power/spell crit or stamina/weapon power/weapon crit.
I think this can be tested very easily, just cast Rite of Passage without medium armor and then cast it with medium armor. If you have similar magicka in both situations but more stamina during the second test you can see if the ulitmate scales off stamina etc.
For example would I as a stamina user be able to use the Templar healing ultimate Rite of Passage to full effect concerning both heal value and crit value?