* SPOILERS AHEAD *

the problem with lasgun and shield is that the center of the explosion could be either the shield, the lasgun or anywhere in between, no way to control it.

Some people are saying it is indistinguishable from a nuke.. that is because it is a nuke, i recall some later novel describing the reaction and basically it cause atom in the air to fuse and eventually blow-up. The effect would happen all around the shield and laser and the explosion could start any place.

But this phenomenon will be used by the Freemen in their attack to Arrakeen to destroy the rock mountains that protect the city from the sand-worms. I think this is show in the trailer, the scene where you see the imperial Sardukars in the ground and then a massive sand worm come out of a sandstorm carrying Freemens, the book even say it is the biggest sand-worm ever seen.

The shield tech in dune block fast moving things, so it not only require a skilled knife fighter, but a fighter that can control their speed, move fast when attacking and then slow down then it is time to pass trough the shield for the final blow, after all it is easy to block a slow moving blow and impossible to hit trough a shield with a fast one. This is supposed be be show when Paul is training with Gurney at the start of the movie\novel but it is hard to portray.

The shield will not block the shock wave from a explosion or the heat because that is just the air moving and the shield does not block air.
As for darts, as long as they are slow enough they will work, they are even the weapon of choice for assassins and Dune universe has "war of assassins" in the landsraad so it is preety much a thing.

As for the freemen, they are the best warriors in the universe because they were raised in the most inhospitable environment that is Arrakis, it has nothing to do with shields, this is made very clear in the novels, and i dont think they even use shields in the desert. They are good fighters because they are raised fighting for very limited resources, so only the best of the best would reach adulthood. One of the things that Paul has to do is unify the Freemen and he has to use the prophecies left by the bene gesserit to do it, this will cause a lot of problems later on.

The second best fighting force in the dune universe at this time (and considered the best pre-dune novel) were the imperial sardukars that were raised in the also inhospitable prison world of salusa secundus. It is made clear in the novel that no one in the landsraad can fight on par with the sardukars, but they are defeated by freemen kids at one point, this is to show how much the Freemen are better then the Sardukars.

Actually, the only reason the Harkonnens were only able to win against the Atreides in the first movie was because the emperor borrowed some Sardukars to then. The only person confirmed in the house Atreides that could win against a imperial Sardukar were Duncan Idaho and Gurney Halleck, but they alone could not win the fight and Gurney was even away at the time.

This is why after Paul became the emperor he moved the house of Corino back to salusa secundus as and geo-enginered the planet into nice world so they could no longer make new Sardukars.