.- Many of these HCOL areas also have strict limitations on AirBnB or short-term rentals... that do not apply to owner-occupants - thus allowing for extreme cashflow potential for house-hackers.

Margins on the market today are extremely tight.

Even the most expensive property managers are EXTREMELY cost effective if you really think about it!

It is extremely odd the previous companies did not ask for that if they are collecting rent for you and issuing you disbursements.  

What I noticed so far is extremely high property taxes. 

Plus the added requirements around getting drawings and general contractor for even adding a tiny closet.

I'm wondering what people's best experiences with low flow are, reliability, durability, best brands, savings seen, and if I should avoid the extreme low gpf toilets for water savings (0.6-0.8 gpf).

You Might actually need to get a night job to go with their day job just to pay for repairs etc on their rentals.I realize there are some people that bought right number years ago in extremely good areas that’s singing the how smart they are song, but that is not the case for most of the landlords I have witnessed.  

Verifying last 2 years of rental history and income extremely important to find the “best of the worst”.Make sure you understand the Class of properties you are looking at and the corresponding results to expect.

Another example of this is Newark, extremely strict laws making land lording a second job.When finding the right area to invest, towns with more lenient land lording laws and proximity to public transportation tend to be a receipt for success.