Although I understand the trolling possibilities Unity offers, I don't think we should focus on what a super small part of the community might be doing.
Unity affecting allies has useful effects on some maps where you can use it on overpowered units (I'm looking at you Soultree). It could be considered game-breaking, but I think it's more about rewarding the use of the correct spells at the right time.
It also expands the cooperation, as you can use it with other players that have less/more hp to move forward in some complex situations.
Overall, I think it's great as it is today. If the campaign "abuse" is too much, I would suggest modifying the units (e.g.: Soultree Viridya) to not be affected by the spell rather than changing the spell itself.