I'm sure it's more complicated than this, but these are my thoughts at first from reading this:

If you iso-raise, you might miss value from someone overvaluing 1 or 2 pair against your str8/flush/set. So if you limp, you want to make a monster against call stations, and can't really bluff.

But raising and getting heads up lets you bluff, and play hands as crappy as mid pair for value. So raising is probably better at nitty tables, and to increase value of hands in general, although you miss out sometimes if you raise Axs and would've had a flush over flush if you didn't.