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.