If someone has a health steroid, like Cho'Gath/anyone with a shield, do you build health or resists? You build resists. If someone has a steroid that scales with health, what do you build? You build health. If someone has a health steroid that scales with health, what do you build? A mixture of health and resists. Health stacking is not the optimal route to tanking things as Mundo. People with free resists? They should stack health. People with damage from health? Stack up some health. People like Mundo and Nautilus, who get health steroids from health? Build a mixture, just like always.
EDIT: I think Mundo is unique in that he has percentage damage with a minimum, though.
And no, a flat addition to percentage damage doesn't count. Go away Vayne.