I've used to think that investing in Heroes was a good idea.
But then I saw that combining units gave you MUCH better "squads" than any Hero could ever provide.
Would take quite a lot of time and resources to achieve, and only applies after several Eras into the game - but in the long run it utterly makes ALL Heroes lacking.
Not to mention that Heroes can DIE (well, unless you have certain cheating Wonders, lol), whereas units can be simply built again.
Time and resources, yes, but still a better overall rating.