Not an app just data, but this is a great website to know what the bottom for rent is for section 8 often times, this isn't true all the time so you have to be aware (it is more true in more populated cities).

Challenges?

It is a tough buy and hole RE market everywhere but markets like San Francisco are particularly challenging in the short term due to the poor cash flow (negative cash flow).  

Some comments on this are harsh, but frankly they are very true.

Labor shortages have hit that industry just like everything else, so finding reliable, trustworthy, quality cleaners is a challenge everyone is facing.

Hello BP Community,I have been a BP lurker for many years and after falling out of love with retail real estate- I am oh so READY to get into investing (my first true love).

It's challenging to distinguish between genuine advice and potential scams in the vast sea of information available online.

Again MAY be true.  

@Robert Frazier Land entitlements are difficult and seems you've now been exposed to appeals and design professional challenges.

If I were to go this route I would clearly identify each of these risks individually, understand how the Govt would potentially challenge each of them and then assess in your particular fact pattern how you would effectively rebut a challenge to each.