Ok, I am usually a road cyclist but ride a mountain bike on occasion during the week. I would like to use the same shoe/pedal combo on my mountain bike that I use on my road bike, if possible. The reason is because I am used to my road cycling equipment and want to retain the pedaling/unlocking muscle memory when I use my mountain bike.
Background: My backyard backs to a well maintained county owned trail (dirt and gravel, fairly flat, not too extreme, I don't ride in the rain and mud). I don't have to dismount and walk while on the trail. The roads around my area are very busy during the week so I tend to train on the off-road trail; its just safer. My mountain bike is a Cannondale F5. I have Look Keo clipless pedals on my Specialized road bike wearing Specialized Sport shoes with Keo cleats. I would like to install these same pedals on my Cannodale F5 so I can use the same shoes that I wear with my road bike.
Are there any technical/practical reasons why this may NOT be a good idea?
Road shoes, especially 3 bolt Look style typically are smooth plastic soled, not the lugged design of a mt. style shoe. That plus the cleats are huge and make walking weird so should you need to dis-mount (terrain and skill dependent) you might have trouble getting up or sown a hill.
As well, SPD or equivalent are easier (generally) to clip in to and that sometimes makes a difference, but again - skill dependent.
I'd put your road pedals on the mt. bike and do a lap or 2 and see how it feels. If it works for you then go for it.
Your concern about muscle memory is unfounded. All pedals, regardless of intended use, move in circles. Unless you have 165mm cranks on one bike and 190mm cranks on the other, there will not be a noticeable difference in pedaling and the pedals you choose have no impact on this. As for unclipping, virtually all pedals unclip in the same manner. I have been using SPD pedals almost exclusively for the past two years and over the past week I have began riding my bike with look-style pedals and the only problem I have found is sometimes I struggle for ~2 seconds to get the pedal flipped into position because I never had to flip the SPDs.
However, there is no reason why you should not use road pedals on a mountain bike if you are riding trails or gravel roads that seldom require dismounting or walking.
Road clipless pedal systems were invented first. They didn't work well for mtbing (walking, functioning in dirt and mud, fast in and out, pedaling not clipped in) so they came up with mtb clipless designs. MTB clipless work much better for road cycling than road clipless work for mtbing. I know many who use mtb cleat/pedal systems on their road bikes.