I have had coaching with mtb coach as well as one that was more "roadie". Personally i think it is important to have the coach in the sport you are doing and here's why:

while someone who does tri or road can help get you fit, a very very important aspect of mtb (or any endurance sport), mtb has a great deal of technical aspects and different strategies for racing than road.

I ended up plateauing with my roadie coach. He helped to improve my fitness, but when it came to mtb racing, i wasn't improving any more. my mtb coach has make small tweaks in my training with huge benefits.

good luck
H