Now that I think about it, the only issues I had with using a mountain bike were the gears being a little low and widely spread, as my trainer doesn't have manually adjustable resistance.
I happen to have a few spare road cassettes (although none go down to 11, boo) so I have slightly tighter gearing, and have let the pressure in the tire down a bit to generate more drag. But really it seems like a great excuse to 1x11 ... ! (?)
Now that I think about it, the only issues I had with using a mountain bike were the gears being a little low and widely spread, as my trainer doesn't have manually adjustable resistance.