They suck, here's why:
Compared to heirloom chest, its 2 intellect vs 4 crit 5 stam, in which even if you don't consider the stamina its 1 int = 2 crit. So if you prefer 1 intellect over 2 crit, then you would get mechbuilders.
But since you prefer 1 intellect over 2 crit, you can get 15 int 6 stam chest, which is 4 int vs 8 crit 2 stam, in which since you prefer 1 int over 2 crit the 4 int would be better.
Sorry if that's confusing, what I'm trying to get at is if you prefer 1 int over 2 crit then your better off using 15 int 6 stam chest, if you prefer 2 crit over 1 int your better off using heirloom chest. Mechbuilder Overall's are in the exact middle, and they really don't make sense to use.