There is a lot of ambiguity in the Soulburst rule, however wether all units (friend & foe)counts, it is very clear, they both do.
The FAQ also says soulburst can not be taken by a unit who's transport (which the unit is embarked upon) gets destroyed.
1. One issue to look out for, is wether you are allowed to pile when using soulburst to fight.
2. Another is if you are able to fight, if you charged in the fight phase due to a soulburst.
3. A further issue is if you consolidated into a new target and wish to fight the new target with a soulburst.
Best way is to agree on these before the game.
My take on it would be:
1. Yes, 2. No, 3. Yes
But honestly the rule is "as clear as sausage broth" IMO.