The invention, advancement, and widespread availability of shield technology has severely limited the effectiveness guns. Conventional projectile weapons became mostly useless and laser weapons became incredibly dangerous, forcing humanity to return to the use of melee weapons.

Two lasguns are actually seen in the film, when Duncan is escaping in a ship and when Paul/Jessica/Kynes are fleeing through the caves. You'll notice that in the former example, the lasgun was only fired AFTER Duncan's ship's shield was knocked out, and in the latter the Sardaukar either knew that the escaping three did not have shields or didn't care if they all died. When a beam makes contact with a shield, it causes an extremely dangerous explosive effect that is guaranteed to kill both the target and attacker (and pretty much anyone else around).

As is shown, this has drastically changed how warfare is waged. To bypass shields, one needs to use feints within feints in order to strike an opponent. In addition to nabbing Leto, it was critical for Dr. Yueh to lower the palace shields in order for the Harkonnens to utilize their artillery weapons. In the book, the Baron muses how clever it is to use the cannons he describes as "antiquated technology" in his attack. The shift in warfare tactics is also metaphorical, as it emphasizes the cutthroat politics of the age.

Adding to the dangers already presented, shields are much less effective on Arrakis in general. Anywhere on the sand, a shield will attract worms and cause them to wreak havoc. Thus, the Fremen also detest shield technology. In the book, Paul's duel with Jamis has a lot of inner monologue about how different the fight is without shields.