The Dune books are a classic case of escalation and overescalation. Herbert frequently introduced more and more powerful abilities for his characters. Sure it may be cool to get a scene where a super-powerful Sardaukar runs crying to the Emperor because his brigades were virtually wiped out by a force of children and women who were left behind in the enemy base... and it may be cool that a 2-year old was commanding those troops. But it strains the credibility, and even if you can accept that, it strains the credibility of the threat. The book ceases to be exciting. So rather than tone down the abilities on the sympathetic side, Herbert adds more to the opposing side.
Ah, you can tell the future? Well, we have invisibility devices (No-fields) that protect us not only from that but also make us completely undetectable through vision, sonics, or what-have-you.
Ah, you are a man who can kill 40 Sardaukar in a swordfight? Well, let's turn up the genetic engineering so that a few generations down the line, in your thirtieth reincarnation (a little broken in itself!) or so, you get beaten up by an elderly librarian.
Ah, there are now millions of reverend mothers? Well let's invent a whole new society of women, the Honored Matres, about twice as fast as the fastest reverend mother, who wield sex like a scalpel to take planetary cultures apart, and when that gets boring they use their gigantic fleets of No-ships to blast the planets to kingdom come. Well, they're taking over a little too fast, so let's create Bashar Teg, a one-man army who can Kick-the-Crap-out-of the Honored Matres, who can KtCoo the Reverend Mothers, who could KtCoo the acolytes if they wanted, who can KtCoo that librarian who in fact did KtCoo Duncan Idaho who became legendary by KtCoo so many Sardaukar, who can KtCoo well-trained ordinary troops.
Despite this, the books are good because the social concepts in them are so powerful (and potentially disturbing, as pointed out by rp in Dune). It is a study of a set of extremes, and the cost of maintaining them. It carries a promise that human potential is limitless (to a great extent even without the genetic engineering, though that helps). It points out many things, which are worth thinking about. But by Children of Dune there can't really be suspense any more.
As for the movie (the one with Sting as Feyd and Patrick Stewart as Gurney Halleck): Weirding devices??? What is this crap? Well, actually, it's a very good rendition of the first third of Dune, with the last two thirds crammed into about thirty five minutes. The sandworm riding scene was fabu, but that's about the extent of what can be said for the second part. The sardaukar were pretty messed-up looking with their little face-plates. How can anyone with such a restricted field of vision be an ultimate warrior? Oh, and Duncan Idaho really got the shaft in this version.