In a med/peds residency you spend a big chunk of time in the hospital and of that time, a sizeable chunk is in the ICU and PICU.
Your training is geared more towards the management of more complex cases in both areas and also inpatient cases.
Someone trained in med/peds is board elegible for both IM and peds, as you know, and can choose to specialize further in either (for instance, pediatric cardiology or adult cardiology, if you leanings were in the cardiology direction). Family med limits your options more.
The board certified family practitioner of today is the GP of yesterday. And there is nothing wrong with that, mind you. You are taught a little of everything and get to do a little of everything too. But, as a family practitioner, you are probably out of your element as an hospitalist, which is where the IM folks come in.
By the same token, a family practitioner will be just perfect with kids who are healthy overall, other than the usual childhood stuff. But if you have a child with some more complex medical needs, you are better off with a pediatrician.
The trade off, as you pointed out, is that you don't get to do any OB, although a lot of family docs, unless they are in a rural or underserved area, don't like to do the OB part, because of higher malpractice premiums.