Just back from a week on Deeside with our Islabikes including a Luath 26, hence the late reply. The Luath is absolutely fine for the mix of road and forest tracks you describe, though depending on the size of the rider, you might want wider tyres such as Marathon 1.5".
That said, my eldest has just moved from the Luath 26 to a Luath 700 with 28mm cyclo-cross tyres and finds it very comfortable.
Hope this helps, but feel free to ask more questions.
Kevin