I have never seen a heart lung shot drop an animal, at least not in relation to a neck shot.
I have shot many both ways and unless (and not often) I have the luxury of watching a deer for 5+ seconds after a heart lung shot expire rather than the normal small shooting window I usually have where I must unleash follow-up shots, I am always amazed in how far a deer can go before expiring.
The cow Elk I took this past season was a neck shot with a .54 patched RB and when the smoke cleared she was on the ground. My hunting partner was watching her and couldn't see me, so the cow hitting the ground before he heard the shot was by his words, very impressive.
I had a low hit once but had I not followed up the deer would not have gone far as I took out one or more large neck veins, but when in dought and have the resource, follow up.