My two cents.
Unless the Assassin Vine can be changed to Ultra-rare (and it sounds like it cannot), then Primal Rift will effectively have a different loot drop table than the other ones (3 epics and 1 ultra rare instead of 2 of each).
I appreciate that the developers are going to open a window to make the troop more available for a short amount of time. I think that is reasonable and I do think that it should include an in-game announcement.
I would hope that the developers will make changes to their processes to guard against another instance of this in the future. Future faction expansions will have to tell the tale of whether or not they make these changes.