I have noticed different team positions have different results.
1 2 3
For example unit 2(mid front unit), if Captain Phasma is your leader, does not really call for ally assists from unit 5(the back unit closer to you on the screen) but often gets assists from unit 2(front closest to you on the screen or 4(back unit further from you on the screen)
I often put Vader in slot/position 4 to get a chance at ability block and Kylo in postion 3 to gain an extra attack for healing immunity.
Position 3 and 4 seem to be targeted more by the AI than other positions.