Nope, it creates imbalance between tribes. However, morale bonus does not. In the end, it doesnt create any imbalance, theres a perfect balance between sarcificing villas and production/rush villas than cata down to minimum pop/go low pop and become hard to cata down.
Fine, then rework the fighting system so the winning side takes no casualties. This does not create any imbalance. It's still an idea thats very obviously trash, despite applying to everyone equally. Just because something is balanced/fair doesnt make it a good idea. That argument I replied to with the brewery example is still bad.
You're right, leaving the ally is also very abusable. I think that a lot of those scenarios could be avoided by making the leaving/joining player isolated for a bit, so others can't send reinforcements to them/they cant send reinforcements to others. Off players couldnt dip in and out then unless they want to get absolutely smashed while theyre in the process.
I think that the ally vs ally is, while still pretty bad, at least in the spirit of the original intention of the rule, and would work better early game. I would like removing the bonus after arts the most though.