But, along with that comes the possibility they will buy some junker, or get some junker for free on Craigslist, that will break down and drain water all over your beautiful hardwood floors......

Tenants will sometimes just use them stealthily.They can buy lint socks for the hose that will drain into the sink or bathtub. 

I did some on the ground research and asked a few neighbors, some of which have lived in the neighborhood for over 50 years, all said they've never had issues with flooding, even in Monsoon season the street flows a bit but is heavily supported by storm water drains and never near the homes.

I can't imagine an inspector would have passed the plumbing rough-in with the tub drain leading nowhere.

Certain cleaning chemicals should not go down the drain and I let them know they should be looking for chemicals marked septic safe.

A french drain is easy (other than a lot of digging), but it needs to drain somewhere. 

The drain to the sink was completely clogged.

You'll never be able to mitigate & prepare for all issues, but you can manage in a certain way to reduce it to the point where it is profitable financially and not mentally draining. 

* Caustic Drain Cleaners: It contains a substance such as lye and caustic potash. These drain cleaners are typically heavier than water, which enables them to reach the clog through standing water.* Oxidizing Drain Cl...

I have been told that they are a pain in the *** because renters will clog the drain and I will get the 12 midnight call.