I currently live in Chicago and own a 2-flat. 

I have been using the flat type for years.

When I found deals they would partner with me and either buy the deal from me and pay me a flat fee, or we would partner on the deal and I would see it through till the end.

In that example, they would be exposed to section 1250 recapture at a flat rate of 25%.

An LLC for your 2 flats or duplexes, another for apartment buildings.

(edited)This will affect investors with separate metering for little used units or devices like pumps or EV charging.... even if you have excess solar generation or whatever the bill charges will still apply. 

Conduct a thorough inspection to assess the true condition of the property.High Vacancy Rates: Excessive vacancies can signal underlying issues such as poor management, undesirable location, or an oversaturated market.

Get all the parties on a conference call.I tried, and believe it or not, title company flat out refused to join a conference call with the accounting firm.  

If you think PITI stays flat, that's naive--thinking it means you're property did not appreciate and net depreciated against inflation.

@Russell Payne there are too many variables to give a flat answer.