Discovering that the enemy army scales has ruined the game for me. Please scale the reward not the enemy army.
I enjoy strategy games at higher difficulty levels because mistakes are costly. By playing well, I don't gain an advantage, I just do enough to stop "myself" from falling into failure and destruction. So I set difficulty levels to determine what level of performance I need to consistently achieve to avoid destruction.
If destruction isn't a threat, because the enemy army will just be weaker the weaker I am, then what am I fighting to prevent? Getting through each battle should be an achievement. If people are not finding the battles challenging enough when they are succeeding, then the answer isn't scaling, it's optimising your difficult levels vs reward. If it's possible to become too powerful before a grand battle then reduce the players income.
Similarly, if I choose to spend lots of money outfitting a unit with expensive guns, then that should be punished by my poor use of them, or because I lack troops elsewhere, not by upgrading the enemy. By upgrading the enemy, all you've done is nullify my decision. If you nullify my decisions then the experience is no longer personal. You may as well just give me a standard army.
So, please don't make my decisions pointless, put some work into setting the difficulty level by scaling the reward, the enemy army should be fixed. Ask players to test the different difficulty levels and measure the number of games in which players are defeated. For example:
Easy = 10% defeated, Medium = 50% defeated, Hard = 80% defeated, Legendary = 95% defeated