Warfare in the Dune universe is an interesting subject. On one hand, technology has advanced quite a bit beyond ours. Most notably, gravity (and friction) nullification seems to be possible (and cheap), they have shields capable of absorbing kinetic force, and they have laser guns. On the other hand, there are several major constraints.
The Holtzmann effect
Any lasgun intersecting a Holtzmann shield causes an unpredictable amount of damage at both ends. Sometimes it will take out both the gunner and the shielded object, sometimes it will be a multi-megaton Nuclear explosion. And it's quite unclear how this works. It is mentioned that Paul (while at Arrakeen) sleeps underground to protect him from assassins suicide-blasting the house shields, but it is unclear why anyone would ever feel "protected" by a Holtzmann shield. Supposedly, the Great Convention prevents the great Houses from just blasting each other to bits, but that feels contrived. It is quite possible that FH intended the Holtzmann Effect and the Great Convention to be a parody on the doctrine of Mutually Assured Destruction.
In any case, since we know that shields are used in combat, it follows that lasguns must be strictly VERBOTEN for anyone who isn't completely suicidal. That means we have to stick to slow-moving projectile weapons (the book mentions Maula pistols) and swords and knives.
I'm not sure there's anything in the book about the amount of force a Holtzmann field can absorb. It's likely that this is at least somewhat proportional to its size, e.g. a personal Holtzmann shield can probably absorb the impact of a round of bullets, but not the direct impact of a crashing thopter.
It's
possible that, say, a railgun or a coilgun might be able to shoot bullets with such a velocity that they can overwhelm (personal) Holtzmann shields, but again, those might not be very practical.
Projectiles in the Dune universe probably bear a greater resemblance to paper airplanes than to bullets. I can also imagine kites,
frisbees, boomerangs, all outfitted with sharp edges, poison, or an explosive payload to compensate for their slow velocity.
Poison gas would be very, very effective in the combat mechanics allowed by the Dune universe, and there's no hint in the novels that any party would be reluctant to use it. The space suits worn by the Sardaukar in the Lynch movie might not be complete bullshit.
The Butlerian Jihad
Man shall not be replaced. No Drones. No computer-guided missiles. Satellite navigation is probably possible, but it's unlikely to be anywhere near as accurate as modern-day GPS. In fact, consider that most modern Jet fighters are fly-by-wire by necessity, and it's unlikely that Ornithopters are more maneuverable (at supersonic speeds) than what we had in the 1960s and 1970s. At low velocity, they switch to a bird-like mode, probably even more maneuverable than our helicopters.
Most importantly: The Guild Peace
There's only one faction that rules space, and they don't want the Great Houses to destroy each other. The Guild does this by making it astronomically expensive to transport troops and military equipment. This means the most economical forms of warfare are, in order:
- Assassins (they probably get to travel at civilian rates)
- Commando troops (if you're going to pay big bucks per soldier, better make them good ones)
- Regular soldiers
- Military equipment (the Guild probably charges even more for this)
House Corrino rules the universe not because it has the best technology, but because it has the Sardaukar; Second only to the Fremen, the best commando troops in the known universe. This allows it to wage war more cheaply than any of the other houses. The fact that House Atreides was able to train a small unit of comparable skill was enough to make it a threat to the Emperor, and seal its fate.
Note that the Harkonnens are not considered a threat by the Emperor. Why? Because their force is based on numbers and artillery, brought to Arrakis at an astronomical expense. In any prolonged conflict between the Harkonnens and the Corrinos, the Harkonnens would have lost.
Extrapolating from this, a lot of "military" vehicles are probably repurposed civilian vehicles. It's probably cheaper to transport a
civilian model ornithopter, or groundcar, and then strap weapons to it, than it is to pay the top Guild rates to transport military 'thopters and cars.
Do tanks even exist in the Dune universe? Why do you need armor when you've got a Holtzmann generator? I can imagine a stealthy, low-profile but speedy groundcar being far more effective. The same logic also makes armored personnel carriers, and even bunkers unlikely.
Airships might make great siege vehicles. They're probably able to carry a big Holtzmann generator, making them practically invulnerable, they move slow enough to penetrate the shields of fortresses, and it'd be very easy to use them to drop bombs or poison gas cannisters on infantry.
Combat is rare in the Dune universe
A thing to remember is that the Dune universe (pre-Paul) is fairly peaceful. Warfare is an expensive hobby for the aristocracy. Even though the books describe great feats of martial arts, the population as a whole is unlikely to be very good at it, and the kind of total war we've seen in the twentieth century is completely unknown.