I have SPD's on both Roadies and my modern Stumpjumper but on all the old Stumpies they have shimano deore or xt flatties.
Havnt used flatties on a road bike for at least 12 years or so. Never would again either if given the choice and they are definately safe on the road. I remember riding a friends bike a couple of years ago with flatties and first thing I noticed was you cant seem to get the legs in the same rythem and definately couldnt climb or power away anywhere near as efficiently.
Have never had a problem on the road but have had the odd embarasing episode on the MTB.
I think when your tired and bonking that you get a hell of a lot further with spds than flatties, I dont know whether its because your in a rythem or whether it takes too much effort to unclip if you stop
My roadies are used all the time with spd's and riding them nearly everyday you soon get used to it, but I find you dont have to unclip very much, especially compared to a MTB on a hilly course. I did around 30 - 40 miles this afternoon as I had to pick something up at the place where I work and only unclipped when I stopped at work and then when I rolled through the gate back at home.
Try them and see, especially if you can get some shoes to go with the ones mentioned above as that way your not investing too much.
My opinion is for the road, it cant be beaten
For the dirt, its a matter of taste I suppose, on rocky stuff I like to be able to dab a foot now and again.
Just remember to adjust the tension before you clip in.