The question you asked is how hard is it to do. The question I percieve you are really asking is how easy or hard is it to do unexpectedly.
The easiest way to learn how to prevent it is to learn how to do it.
Sometime when you have righting assistance, take the 18 out in a blow, set it up on a screaming close reach, and then drive it down while holding the main, or even sheeting it in a bit.
If its blowing hard enough, Voila, bow stuff, and possibly/probably pitchpole.
Once you understand how to make the boat pitchpole, it is relativly easy to avoid that particular manuver and avoid the pitchpole.
A bit of a story to illustrate. When I first bought my 18, back when the dead sea was only sick, I went out with an experienced cat sailor, and was bragging to him that with the tall bows and bouyancy, the 18 was almost impossible to pitchpole. His reaction was to scoot forward a bit, sheet in and drive off suddenly and hard until the bows stuffed all the way to the daggerboards. Scared the bejesus outta me.
I learned the lesson and in 28 years of sailing the Hobie 18, I have been close 3 or 4 times, but only actually pitchpoled twice. Once in a 45 knot blow here in Colorado while trying to get back to the Marina, and once when sailing @ the H-18 Worlds in Melbourne. Keep in mind that most of my sailing is inland lake sailing where I don't often get into wave action over 2 or 3 feet.
To try and avoid the pitchpole on the H-18, sheet out, and head off gently, if the wind is blowing too hard, and the bows are burying, or the boat feels unstable, sheet out to depower, and head up. The H-18 is one of the most forgiving boats in this regard and really quite easy to keep on its feet. Most of the times I have gone swimmiing unexpectedly are caused by moments of inattention where a wave or a puff catches me by suprize, an equipment malfunction, or just an error on my part.