Of course to maximize your tax benefit it needs to be sustainable over time. 

Mixed-use properties that integrate residential units, co-working spaces, and retail areas with sustainable design elements are particularly appealing.

The 15% return is in paper and not sustainable. 

Certainly even someone of your intellect must understand that if you're lying all day in order to promote investments that you do not have a sustainable business model.

While I use it primarily for noise concerns (it alerts me for sustained +80 decibels), it's also handy for cigarette smoke and tracking temperatures (in addition to our Ecobee thermostat)- sensors on all exterior doors and possibly windows (which you might have via smart locks for the doors)- exterior camsA recent guest for reasons only known to them set the thermostat to cool and 60F in the winter (where it's currently a balmy 11F). 

Otherwise, his rental portfolio is sufficient to sustain him.

Investment also spiked in Class A Short-Term Rentals (STR) and investors started paying higher and higher prices based upon anticipated STR rental rates, that exceeded sustainability based upon Long-Term Rental rates (LTR).

However I would sell these units as $500-550/month rent will never be sustainable.

While this may be risky in other ways, you'd be able to continue to grow your portfolio without taking as much equity from your income producing properties and possibly sustain your continued cash flow with them. 

My advice would be is to talk to multiple investors who have real world, actual success-based experience in flipping over a sustained period.