There are much more ways to increase the happiness cap than there are ways to increase the health cap. While unhealthiness is a less serious than unhappiness, beyond the very early game unhealthiness more often than not is the real limiter on city size.
This is correct, but I have never found it to be crippling so far. Maybe this is due to my play style (which included neglecting GP farms up to this point, one of the many weaknesses of mine). If you look at it, there are quite a few sources for health points even in the medieval era: 11 health resources, 9 of which are common (banana, clam, corn, cow, crab, deer, fish, pig, rice, sheep, wheat) and granaries, harbors, and grocers, which provide further health. Later on, when your cities grow really big, you get additional techs, improvements and the Environmentalism civic if you really need it.
So still, I'd like to see a convincing scenario where health was really the showstopper so the +3 bonus would have helped a lot. And I'm not talking about ridiculous difficulties or the OCC.
Probably, one of these scenarios involves your civ growing mainly vertically, but then your entire strategy lends itself to an expansive leader anyway. My point though was that a killer trait should be versatile and work well for all kinds of strats (like Financial does, in my opinion).
--Sigi