You might also want to think about cast. One reason most people ride offroad with a helmet light as well as a bar light is that when a light is closer to the ground, shadows from obstacles are longer / bigger. Ideally you want a light fairly high up. Typically you see under bar set ups on road bikes, where there’s less in the way of uneven terrain and a big shadow is actually useful for avoiding stuff.
Oh, and as above – I’ve been riding off road for about 25 years pretty regularly, and I’ve crashed a couple of times where the light has been in the way. A couple of scratches on anodising (Lumicycle) or broken lenses (Halfrods D cell halogen monstrosities in about 1994/5) and that’s it. Top mount your light, enjoy your ride. 🙂