If they are VERY professional, they will have their processes in writing as verification that policies are enforced equally and fairly by their entire staff.6.

Some things I do to avoid bad situations: screen well, only have properties in good locations where people actually want to live (I find tenants behave better when they know they have a good thing going for them and that they are easily replaceable), set proper expectations, have a rock solid lease, go over it with new tenants to make sure everything is clear and agreed upon, and enforce the lease by issuing a formal notice to pay or quit every single time rent is late by even one day (and charge the late fee), or a formal notice to cure or quit every time there is a lease violation.

In regards to point 4), you mention the possibility of a tenant living for free indefinitely inside an unwarranted unit, due to the inability of the landlord to enforce rents, and the inability to perform an eviction.It's my understanding that if the Department of Building Inspection (DBI) discovers the unwarranted unit, it's possible for them to force the landlord by law to legalize the unit.

If they are VERY professional, they will have their processes in writing as verification that policies are enforced equally and fairly by their entire staff.6.

-Get to know local code enforcement or inspectors.

They are enforcement actions of a legal contract term, executed per the prevailing laws for terms enforcement.

Who would enforce the covenants, the city or county? 

Quote from @Tyler Battaglia: Forms and contracts are important, but you need to really understand them and know how to enforce them. 

They rent below market rates, they pick bad renters, they don't have a good lease or know how to enforce it, etc.

Even small, common mistakes like renting below market, picking the wrong tenants, unwilling/unable to enforce lease terms, slow turnovers, or unscrupulous contractors can cost more than a good PM will charge, and that's before you account for your time and the stress.If you have the personality, I recommend self-management for anyone who lives within 1-2 hours of their investments.