Let's take a marathon race as an example. I wouldn't stand a chance in such a competition. Now, let's modify the competition – give each participant a car. Suddenly, I'm on equal footing with the other runners. The point of the competition is lost because the cars negate the difference between us. Similarly, mass Slow negates the difference between players.
Just as a car doesn't require physical strength to operate, mass Slow doesn't require the player to have technical skill in conducting battles. It is battles with mass Slow against powerful units that can be described as dull or mindless. All other magic requires you to use your brain, making battles more interesting. And only mass Slow turns a battle into a boring, tedious shootout with no losses.
The advantage that mass Slow provides in battles with neutral creatures is enormous. However, there is one caveat – this advantage decreases as the game progresses. By the fourth week, it usually doesn't matter whether you have Slow or not; there are enough troops anyway to defeat neutral creatures with minimal losses.
As for the resources spent on obtaining mass Slow, luck plays a bigger role than resources. After all, building the first level of a Tower doesn't require many resources.
In <might and magic>, the problems with mass Slow (I'll remind you – there are two: boring, tedious battles and a large advantage with low cost) were solved by moving Slow to the second tier. Thus, obtaining Slow in the early stages became more difficult. This requires more resources and time. On the other hand, the influence of luck on obtaining the spell in <might and magic> has been reduced, because any floor of the Guild of Mages can be rebuilt until the desired spell appears.
Nevertheless, I believe that the ideal solution would be to make mass Slow a separate spell on the fourth tier, as it was in Heroes of Might and Magic II. That's where it belongs in terms of the ratio of effectiveness to the difficulty of obtaining it.