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Humans... what's the argument about?
Let's break it down:
We're discussing luck and morale in Heroes 3, where they have roughly the same effect, with slight nuances.
Hitting twice and hitting with double strength – in terms of damage, these are practically equivalent options (for Heroes 3).
The better the morale, the more you can change the action (hit another target/retreat/go on the defensive) or, at worst, inflict the same damage.
Luck is better because it inflicts double damage instantly, i.e., before the monster's response, which leads to all sorts of consequences...
Here's an example: 50 blessed harpies attack five Black Knights with luck, either once or twice in a row.
In the first case, they will deal maximum damage with a double hit (200%), and in the second case, when the Black Knight responds, a curse may trigger, and the 2nd hit will be 4 times weaker (125%).
There are many such examples, but not enough to consider luck more effective than morale (in general).
P.S. I think the developers took this into account, including the fact that the Leadership skill (the main source of morale) is found more often than luck, and is even a basic skill for the entire line of "Knight" heroes