Even if you stack straight HP items on champions that benefit from them and even if you farm effectively and even if you manage full build, Overgrowth will still only give you somewhere between +3% to +5% maximum health. That amounts to +30-50 health for every 1,000 maximum health you have. So, if your champion ends with 4k to 5k maximum health by the end of a match, that's a mere 120-250 bonus health from a rune and that's only AFTER a rare instance in which you play a long game at which point most champions that even bother building tank end up shredded by late game hyper carries.
Overgrowth provides no benefits early, mid, or late as a result. It's just plain terrible and the only conceivable reason why anyone would bother with it is if they are capable of using Grasp of the Undying consistently so as to stack health not just off items, but also their keystone.