As you can imagine that meant that we had a lot of trouble makers and a generally undesirable tenant base. 

Buying a home in an undesirable area "can" get you undesirable results.

I have one fenced because undesirable tenants in a duplex behind my property.  

I think buying these "undesirable" properties is very risky for most investors and may be money down the drain. 

In having iit at 7:30 am, you will screen out 90% of all the undesirable tenants who party hard and are irresponsible, whereas anyone who will wake up that early will be a very responsible tenant who can rent long-term. 

After the crowd of undesirables were gone, our tenant was still angry with us for intervening.

It’s a method to sell otherwise undesirable houses In  Sketchy areas to the locals that can’t get  traditional  loans .

I would then use Trulia's crime heat map function to filter out any 'undesirable' areas as well.  

In no way was I implying the tenants of these properties were bad people, simply that when I tour the property and talk to them there are extenuating circumstances and strange deals with the current landlord that have allowed them to modify the property and use it in undesirable ways. 

To get 12 -15% in today's low interest rate environment requires one of three variables (1) work the note - usually a non or sub preforming note or (2) accept a higher risk, i.e, second lien position, very high LTV, borrower with very low credit scores or bad credit history, D property in undesirable areas, etc, or (C) get VERY lucky!