Hero engineers.

Besides machines, what else do we upgrade? Are there any specific features when leveling up these heroes, considering they are machinists, or do we level them up like other heroes, such as knights, demons, etc.?

Logistics and the Path of War are simply essential for machinists, even at the expense of everything else. This is simply because leveling up a machinist doesn't depend on external factors, but only on the hero's movement speed. In this regard, Grog is very appealing, especially when he can acquire machines early on.