The Atreides were both surprised and overwhelmed.
They knew an attack was coming.
The big surprise was the scale, which overwhelmed them. Thufir had predicted 1 legion (30,000 troops), but the Barron landed 10. The Barron stated it took 60 years income from Dune to pay the Guild shipping costs at hazard rates.
The second was the involvement of the Sardaukar (2 legions). The appendix states that they were each the equivalent of 10 normal troops, but it is unclear if that was at their peak, or the time of the assault.
Finally, there was Yueh brining down the shield around the HQ. Suk conditioning was believed to be unbreakable, so he would have had the highest security clearance. That make the assault easier, but with such numbers, it is hard to see then loosing even without this.
So up to 30 times the expected force, plus treachery.