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I used to do the same thing – I would build all the growth structures first, and then upgrade, but I realized that it’s not efficient. Let the growth accumulate, but you’ll lose even more by fighting with un-upgraded units in the meantime.
The randomness in Heroes 5 is just crazy. In a scenario for the Demon faction, I got a very rare light skill early on, but in a scenario for the Warlock faction, I only got Chaos, which is one of the most frequently offered skills, on the eighteenth level.