The very small clearance doesn't stay in adjustment very well, and the cam and the bottom of the lifter has a tendency to wear pretty quickly and increase the clearance. It's just not designed to have any clearance at all, as the "shock absorber" action of a hydraulic keeps the lifter in contact with the cam at all times. If you try to adjust for zero clearance with a solid lifter on a hydraulic shaft, the valves burn because of pushrod and valve stem expansion as the engine gets hot, which pushes the valve off the seat.
Jerry
