My experience so far has been positive, but it's early days.

Mostly people with minimal RE experience are adding a single ADU and they often make that choice based on cash flow without taking into account the negative equity position.  

You're in a good position, you just need to figure out which option works best for you. 

If you used a loan to rehab your property and find yourself in a difficult position, doing a straight refi (just paying off the old mortgage) will be easier than trying to pay off the original loan and take additional cash out.

that we’ve learned in our 24 years, managing almost 700 doors across the Metro Detroit area, including almost 100 S8 leases:Class A Properties:Cashflow vs Appreciation: Typically, 3-5 years for positive cashflow, but you get highest relative rent & value appreciation.Vacancy Est: Historically 10%, 5% the more recent norm.Tenant Pool: Majority will have FICO scores of 680+ (roughly 5% probability of default), zero evictions in last 7 years.Class B Properties:Cashflow vs Appreciation: Typically, decent amount of relative rent & value appreciation.Vacancy Est: Historically 10%, 5% should be applied only if proper research done to support.Tenant Pool: Majority will have FICO scores of 620-680 (around 10% probability of default), some blemishes, but should have no evictions in last 5 yearsClass C Properties:Cashflow vs Appreciation: Typically, high cashflow and at the lower end of relative rent & value appreciation.

You’re in a strong position with multiple viable options.

However, one of the other REITs I invested in quadrupled in price, and as a whole I’m $150,000 positive.  

This has pluses and minuses.If your property is bulletproof such as no framing - cinder block construction, cement floors, gloss white paint slapped on the walls and a cheap window mount AC, then you are in a prime position to make money with S8. 

Are you generally seeing cash flow positive deals from day 1?

The 1st position loan is paid off but has a second position heloc on the home with 50k balance.