Braken fern is the most commonly eaten fern. However, it does contain a mild toxin. So I wouldn't make a steady diet of the stuff. I have one fern 'Licorice fern' whose rhizome has a licorice flavor. I am not aware of any specifically poisonous ferns.

Ive heard of eating very well boiled fiddleheads as a survival technique when lost in the woods. I dont recall if the boiling is to reduce toxicity or make the nutrients more available but I do remember that the boiling is supposed to leave a very slimy treat!

Braken Fern can be harvested when the head is tightly cured. With two fingers slowly sweep upwards with slight pressure when it will snap over. Must have everything ready before you start. Pick a nice sunny day that will dry the greens. If you have enough(about 2-3lbs),get a pot a boiling water and blanch for about a minute,turning once. Scoop out of boiling water and lay it outside in thin layer to dry.Hot braken will dry very fast.Turn in every 2-hours to dry completly. When it is dried, Soak in water for few hours and changing water each times. About 2-3 times. Rince gently. Can be stir fried. or in soups. Tasted like mushroom with a slight meat overtones. Lot of work which I would not bother unless I have large quantity. Worth the labor. One of my favorite dried greens. Soaking takes all the toxicity away. My mom stir fries with olive oil and lots of garic and little bit soy sauce and salt.

Mmmmm, that sounds great!