Tactics for playing as the Dwarves.

I don't know how useful machines are for Gnomes; I've never used them.

You should try them. Machines are a real help for Gnomes, even without the skill. With high unit defense at the beginning of the game, even 10 hit points of regeneration is noticeable. The ballista is even more noticeable because each battle is prolonged due to the low damage of the warriors, and the cart is beyond comment... Besides, it's very easy to acquire machines (15% chance), and Gnomes, in addition to the triple ballista that has become standard for everyone, also have rune machines (+3 to machine initiative). Gnomes also need attack, so taking fire arrows is not a problem. As a result, we have a ballista that is only slightly weaker than those of Demons or Orcs.
However, there are more interesting perks for Gnomes instead of machinery, such as defense, leadership, luck, some kind of magic... that's why they are not taken so often...