probaly reason for forbiding tech is silly suggestions of gnomes and usually going too much in quantity of gears, clockwork and steam to be unbelievable.
so some tech could go in it i guess like primitive guns (great damage but in game lower range because low accuracy of first guns and low ammo for huge reload, maybe 2-4 shots for normal units as archer more ammo for cannons as example of gun tech before playtest), but maximum is "little" under tarantula tank from wild wild west and mechanical dragon in heroes of might and magic 4 as some most extreme "don't do it" example?