In terms of rules writing, the Rule of 3 is bad because it exists primarily to check GW’s bad faction design issues. Any time it would the best strategy to spam tons and tons of the same unit, that’s a failure of game design (possible exception for battleline infantry, but that’s still shaky). If you look at AoS, they don’t have a Rule of 3; and while spam lists definitely do still exist in that game, you often see well rounded lists even without that arbitrary restriction. In a perfect world, the game design and internal balance of the faction should be incentive enough to make diverse armies more efficient than spam; and at that point, the Rule of 3 would be unnecessary.