I used to wear monthly contacts all the time. Now I wear glasses, but I have disposables pretty much exclusively for the bike.
You need to wear eye protection as on fast descents the wind can dry the lens slightly and this can result in slightly blurred vision, also if you get mud in your eye it can be quite uncomfortable. If the eye waters too much then you could find the lens drops out.
Wearing contacts in the rain with eyewear gives much better vision that rain on prescription glasses. I won't ride in the rain without contacts now unless I have no choice.
Edit: I also have astigmatism, used to cause problems when I had lenses that didn't fit quite right, but with the dailys I have now, as long as I have eyewear on, they are great. Any lens will blur if it moves off-centre, but with astigmatism, it will also blur if the lens rotates. The lenses are thicker at the bottom to keep them at the correct orientation, but if the eye dries out then lens can stick at the wrong angle.