Flat tappets are limited in the steepness of the 'ramp'. ( how RAPIDLY the valve will begin the opening process). Rollers can use an almost vertical ramp simple because of the design. Example is off road 4X4's going up an almost vertical rock because the tires/rollers are rotating instead of sliding. Rollers have a very small contact 'patch' as opposed to flats which have to have a larger and offset contact to promote rotation of the lifter for lubrication/even wear action.