Really stalls piston motion B/ATDC, initiates vacuum later, reduces piston to valve clearance. This is acceptable for an engine with small ports + large displacement (or no need for low speed power, like F1), but kills torque if the engine size drops. The opposite (big ports and short rods)? BBC, works very well.
Reducing engine size by bore change is almost harmless as long as the new wall position does not significantly shroud the intake valve. By comparison, reduction by stroke change, even with reduced deck height, seldom does what you want.