They work well in flatheads because we have to reduce the base circle of the cam so much not to run into everything. The extra length of the billet tappets makes it easier for the builder not to have to weld on the valve stems. As far as OHV engines, that is a different conversation. You want to make sure the face of the tappet is the stock size or at least the size that the cam was designed for, if it is different than stock.
Mike Edwards