My theory is based on the kishi's emphasis on the purity of the soul as a prerequisite for being the perfect host... (case in point being the 8 tails host before killer bee and the whole rhetoric about erasing all hate from the soul - falls of truth chapter etc.) so to prevent loss of self, a...
that's the thing about the sharingan... its activated and enhanced through the pain of the user... you obtain MS by killing a best friend... you get EMS through pain of losing a brother (also the fact that you get MS+MS)... thats why the uchiha history is tainted with blood and all that jazz...
I doubt that there's any difference in the rasengans intensity... in fact nautos should be stronger considering bijuu mode etc. Maybe the perfect body theory
I was just wondering... if you can get the ems by taking your brother's eyes, why can't brothers mutually exchange their sharingans... the fact that izuna was blind makes me think of whatever the f*** happened to madara's eyes after he took izuna's... you could argue that madara was practically...