Road running and trail running are dramatically different skills. Road running requires using the same gait for a long period of time. You're stressing a relatively small number of muscles repeatedly. Trail running requires a much more dynamic gait. Because you have to jump around the trail to avoid obstacles like rocks, roots, and water, you stress a large number of muscles in various combinations.
When road runners run trails for the first time, they're often struck by the difficulty of utilizing different muscle groups. The same phenomenon happens when trail runners run roads for the first time.
If you train in conditions you'll likely experience during a race, this is not an issue. However, I would advise any runner to add at least one run per week on a different surface. Road runners should occasionally hit the trails. Trail runners should occasionally hit the road. It will dramatically enhance your ability to switch between the two.
For my first 50 miler, I trained almost exclusively on roads. The trails killed me. Over the last few years, I've shied away from road running. Whenever I run a road race, it takes much longer to recover.