Just remember, with every buff you give them for flavor, you have to increase their cost to compensate - an eight movement, flying, 23 strength Magic Immune unit that has Roar?
That needs to cost a LOT of hammers.
Why does it need to cost a lot of hammers? The hammer cost for pretty much all heroes aside from meshabber/mythril golem is really irrelevant
The cost for them comes in the tech. Acheron is probably the easiest to get, as you can capture him with beastmasters at Animal Mastery. Coatlann needs Arcane Lore, Theology, and Righteousness, three fairly late techs. And Drifa, eurabatres, and Abashi, all require divine essence.
If you can manage to tech that far, the hammer costs are pretty much irrelevant. Hell, in my current Mazatl game, my capital city has about 350

production per turn. And that's with almost no mines. IIt's a financial city. If playing a different civ that doesn't have deep jungle everywhere, I'd pack every available tile with mines/lumbermills instead, my production could have been much higher, even.
Meshabber is reduced to what...1200

is it now? that's 4 turns of work, if I decide to actually build it. More likely, I'll just use the huge bank balance I always have saved up for exactly that sort of purpose, and rush him 1-2 turns anyway
The game is usually won by the time divie essence comes around, in any case. It's one of those points where game balance starts to become irrelevant, and the game's mostly about destroying the world with your shiny new toys.
"an eight movement, flying, 23 strength Magic Immune unit that has Roar" is a powerful thing to be sure, but so are 4 archmages, 4 paladins, 4 immortals, 4 phalanxes, 4 berserkers, 4 knights, 4 high priests, your choice of religious heroes, and however many musketmen you desire to back them up. That sort of military power is not hard to come by when you have several cities that can make almost any unit in a single turn, and compared to that, no matter how powerful they are, a dragon is just icing on the cake.