  On H-type sub-Riemannian manifolds we establish sub-Hessian and sub-Laplacian
comparison theorems which are uniform for a family of approximating Riemannian
metrics converging to the sub-Riemannian one. We also prove a sharp
sub-Riemannian Bonnet-Myers theorem that extends to this general setting
results previously proved on contact and quaternionic contact manifolds.
