Well if your using a 'Silvertop' block you will have problems finding the timing pin location.
So your not logging either valve opening or closing when timing.
Vernier pulleys have always been an important aid to timing. Necessary back in the days of keyways (though they could be left out) to correct minor machining inaccuracy before CNC grinding. Nowadays they are necessary for optimising timing on a dyno. You can't tweak cams without an indication of movement.
With aftermarket cams the accuracy can usually be found to be good on the slots, its just the optimal setting might be 3degrees out. Found that to be very consistent on several dynoed engines.
Fords own cams are often damaged by owners who've used the alignment bar as a lock bar. And there is known tolerance problems which led to TSB issue.