I agree with Mike H. that 50-60 homes while working full time is achievable without ruining your life.

Stains, pets, bubble gum, irons, pest infestation, or anything you can think of will ruin it.When I still had decent carpet in some units I acquired, many people would ask, "Are you going to pull the carpet up?" 

Ruined all the laminate floors, pitbull chewed up all cabinets and to top it all off, his cat got stuck in the toilet and obviously died after several days.

Vacany and poor tenants ruin your returns not to mention potential damages to the unit. 

Then I would call the parents with your concerns(sob sob) about not wanting to ruin their kids credit at 22 but you need to get paid or take action. 

Unfortunately, there are people with nothing better to do that try and ruin others work.

Passive-aggressive will be taking their anger out due to having to fix this problem by not warning you of a steady drip in the water heater until it explodes and ruins the flooring in the adjoining rooms.  

TENANTS ARE ****** PEOPLE AND THEIR PETS WILL RUIN EVERYTHING ESPECIALLY.  

I have heard about immature children ruining their parents credit and doing crimes etc.

I know the practice is fraught with issues and ruined friendships.