I think it's already clear... The extra growth of level 7 units can certainly backfire :) For example, in the rules of the past World Championship, as far as I remember, they were either banned from being purchased, or you simply had to load a previous save and restart the week... But in the rules of HeroesWorld, there is no such provision, so you have to discuss this with your opponent in advance, although such cases are rare..
And the best option is to play HT - there all weeks and months of monsters and plague have been fixed:)