However, there are a lot of undesirable areas here.

I am now a grumpy old landlady who can't seem to keep up with this ever growing pool of undesirable tenants who know too much!

The wholesale properties have either been too expensive, or in an undesirable area (I don't want to be a slum lord), or in a location too far away. 

Immovable undesirables (landfills, sewage plants).

In undesirable area's it is easier for you as a landlord to find section 8 tenants because you will be paid the rent on time by their voucher provider.

I have bought a few what i call Tweaker parks.. you know full of drug addicts ,, convicts and other undesirables.. 

The rent will be much lower so you'll need to make sure you buy right in order to get the CF you desire based on comparable rents.The rents and tenant pool should be a concern but exit strategy should be your main concern buying a home that would be fairly undesirable to any family with a size >= 3.  

That should protect you if in the future some other undesirable comes along and they try to use the defense that you allowed it before.

Undesirable lot?  

This is actually incredibly important in places like NYC where you can buy an Investment property that you think is great at a reasonable Cap Rate but failed to see that it is right next to a high crime or undesirable area such as next to a homeless shelter.