Many judges would feel you got paid even if no cash was sent to you.I could see this potentially working on a higher-end property that it in excellent condition with a Grade A tenant.  

.- 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.

Also if you have significant wealth, using a good PM mitigates potential risk and headaches.

I see potential hurdles when it comes to resale but who knows what the future holds for these type of properties.

A second STR in a market with year-round cash flow potential might balance out your portfolio and give you more consistent income.

If you are organized enough to forsee hang ups and talk with your GC about potential challenges on a regular basis, two brains are better than one.

I have a pretty large emergency fund in place for those properties, but I wonder if that capital could be better employed elsewhere (potentially acquire another property).  

One potential mentor I had one phone call with and never contacted him again. 

If I put myself in the shoes of the outside partner/investor friend, I would be very concerned because: 1) unclear and/or potentially conflicting business plan, 2) even with clarity in the BRRRR business plan, unclear in the viability of pulling it off with these market conditions, 3) the chances of dispute seems very high, especially if you're going to live in. 4 years ago, I went through a  similar thought process when purchasing an investing property with mainly my family money, while considering an outside investor to rehab for higher STR ROI... 

All major flippers in town have heavily compressed ARV to accommodate for the potential of the downward slide to continue.