In my understanding, it really depends on the area. And probably more than that, it depends on your goals.
In more stable areas, you are more likely to find lower cap rates (which means less cash flow), and in less stable areas, you are more likely to find higher cap rates (which, of course, means more cash flow).
It's not all bad, though.
(Excluding <5 unit properties,) The total value of your multifamily property increases as cap rate decreases. So an apartment complex in Tampa, for example, that cash flows 2% of its value per year, doesn't really flow what a complex in downtown Detroit might flow (let's call it 8% just for giggles). But it will likely have a much, much higher value. It's a much more stable asset.
On a personal note, my cash flow end-goal is $10,000 per month. If, after analyzing the financials of a property, I am confident that it will always cash-flow positively (because the debt is long-term, the rate is fixed, and the balloon is either inexistent or super long-term), then it helps me reach that goal. I'll have that property, my kids will have that property, and if I teach them well, maybe they'll teach my grandkids to have it, too.
So for me, I probably give the cap rate a quick look, and if I know the cash flow is within 1-2% of this "expected" number (and again, I am confident it will always cash-flow positively), I am personally not worried about per-door cash flow, because I can manufacture that by improving the property, and...well, getting paid to wait, because I chose my market carefully.