LBE – minimal balance changes for Heroes V 3.1.

In the early stages of the game, the ballista and tent are undoubtedly extremely important, but after a month of gameplay, their usefulness decreases significantly.

Of course, it decreases. Because initially, it's disproportionately overpowered and greatly helps in development.
Therefore, by investing in machines early in the game on a map without a mentor, to have an easy start, the player consciously takes a risk if the game progresses into the late game. This is normal.
Even if, somehow, it becomes possible to make the strength of the machines more proportional to the hero's level/the passage of time, significantly weakening them at the beginning, it will be extremely boring.