MTB and SPD dangerous?

Clipping out of the pedals gets to become an instinctive reaction ... mountain bike pedals don't need as much force as road pedals to unclip from.

There is another issue of pedal performance in the mud .. not exactly a strong point for some SPD pedals There are other MTB pedal systems (Time ATAC, Eggbeater, ...) that are better in this regard.

I just made the jump to SPD pedals on my MTB, have been clipped into my road bike for some time.

The SPD pedals unclip a lot easier than the Look pedals (I run Campy on the road), because you need to unclip a lot more often. Just a flick of the heel and you're out. I get the occasional accidental unclip, but that's better than not unclipping when you need to - and I did that a few times, too. Getting into the double sided pedals is a lot easier than the single sided, tilted upwards Campy pedal.

One very nice thing: you can walk around in shoes with SPD cleats, no problem. I suppose you can walk around in shoes with the Campy or Look cleats, if you don't mind stumbling all over the place and sounding like a pile driver.

You don't actually put your feet down too often off-road, but like other people have said here mountain bike pedals are easier to release.
You might consider one of the pedals which have a full cage, with a double sided clipless core. These can be ridden like normal pedals without the need to clip in. They can help on technical sections where you might not be happy clipping in. The Shimano 646 DX pedal is a very good example of this. They weigh a little more than normal SPDs but function very well.

i race downhill and i use SPD's i have no problems unclipping but i have the SPD's done up as tight as they go. i rarely unclip in a race and find i have more control through corners. i think they are a great idea for MTB's.

NOTE learn to ride on flats first otherwise you will rely on you SPD's for bunyhops and other things.(very bad habit of roadies)

I had 2 stupid off's due to spuds on my MTB. The first was the day I got them. I was setting them up in the yard, very mindful of the fact that I was clipped in and so leaning the way of the free foot. When I changed over though, I leaned the same way as before and fell on my side (bike on top of me feeling a right ****) hoping that no-one was watching!!!!!!
The other was when I was tired and I forgot to clipout approaching to climb some midforest steps, again falling on my side.
Since then I've had no problems as they become natural just like using brakes to stop, you clck out to put you're foot down.

If it's looking precarious I normally click out and pedal using the middle of the shoe on the spd. It's not exactly efficient and there's no control but it saves a grazed face on the tree's.
I have North Wave trail shoes so I have the grip to click out and pedal.

B

Also for the learning curve you can adjust the tension to make clicking out easier when you start on the flats as suggested above and slowly increase the tension as you grow more accustomed to the spds off road.

I ride with "platform on one side and SPD on the other" pedals. Choose your brand. I rarely worry about being clipped in on XC type riding, but if you want to try sketchy technical and are not experienced, platforms give you more chance to dab rather than face plant. Knowing you have the option to ride the OHMIGOSH sections with unencumbered feet lets you stretch the limits. IMHO