As an example, if you're driving your car down the road with your high beams on, cars will flash their lights at you and honk and maybe even wave with one finger if they're feeling extra nice. Can you do that? Sure, but you won't make any friends that way, and more to the point, it's less safe for everyone involved.
If an oncoming driver can't see anything, then they definitely can't see you.
So in this way, yes you can use a mountain bike light on the road, but they will produce a lot of glare for oncoming traffic, which is not ideal. Our road-focused products have horizontal cutoffs in the beam, just like your car's low beam headlights, which has the benefit of putting all that light where you can use it (on the road instead of into the sky) which means we can use less power and get longer runtimes, while also preventing dazzling traffic. You can still see cars coming out you if just the low beams are on, the same applies to our road products.
We design each of our lights not just for a specific type of riding, but a specific mounting location, as that determines what kind of beam pattern is needed for best performance. On singletrack, the terrain undulates up and down constantly, so a cutoff beam pattern constantly pitching up and down will not only be distracting, it will leave half the trail missing a lot of the time when the going gets rough, so a road-optimized light is not great for the trails, which is why we design beam pattern for our trail lights to be much wider spread with no cutoff, so that you never lose the trail.
Because of this, using an MTB light on the road you will be spraying light into the eyes of oncoming traffic, which makes it harder for them to see you, which makes it less safe for everyone. You can do it, but horses for courses, we always recommend using the right tool for the job.