One was a recent purchase and the latest owner didn't maintain it that well so I had a few items come up in one inspection that needed repair.  

I do not touch any of my profits from my properties ( unless they get up to a steady amount, usually 10K for me) this way that money is there for a new roof, furnace, hot water heater, whatever may come up. i just purchased a place and the first thing i did was have the roof replaced, 15K later, i know i am good for at least 25 years, if a storm does not rip it off, but now i know that property better have at least 15K in reserves. take what will be your worst case repair and make sure you have enough, or close to enough, in reserves for that property. but as others have stated, constant inspection of properties and equipment will help you prolong any of that, have an HVAC co. check the systems or a plumber check and maintain the furnace either every year or every other year, check roof for missing shingles or exterior for rotted boards, keeping the water out will maintain the structure within, exterior maintenance is a must.

These aren't childhood friendships you are trying to maintain, this is a business dealing that requires legal remedy.

Most Counties maintain a database with such properties.

Easier to maintain.  1 roof instead of 3, 4 exterior sides to maintain instead of 12.... you get the picture.

People struggle with setting priorities and maintaining commitment.

I would say they are aggressive but there are also "slumloards" that maintain despicable properties and have no conscience with respect to their tenants. 

She worked for the University, so literally got free tuition if she maintained a certain GPA. 

You want to do those renos where someone will appreciate them and maintain them.

I.e. keep the grass mowed, have some lights on timers (esp exterior lights), and generally keep it looking clean and well maintained.