The big thing mentioned is the unknown of future regulations and "hotel taxes" from cities (always looking for more money any way they can!)

I personally would never even consider this. 1) I'd never want unknown people walking in and out of my properties whenever they want. 2) Meeting them in person is a MAJOR part of my screening process.

High humidity in the kitchen from lots of boiling can also encourage this.A bigger concern for a landlord is the unknown factor; i.e., a pipe in a wall leaking. 

Every where residents opposed to STR, it has been usually due to fear of unknown, assumptions, rather than real disturbance.

They knew they had an under market rent for years with an absent landlord, and were happy to pay still slightly under market rent for their home.4) Remember that turning a unit with a long term tenant is typically at least 1 month vacant (as it can be hard to show units with long term tenants), painting costs, and I would add in up to another $1k on the unknown issues.  

Their reserve is still unknown.

It's my understanding that we're allowed to screen everyone the same but I run into 2 problems with many of the s8 applicants:1) They generally can't seem to tell me how much their portion of the rent/utilities will be (which leads to #2) 2) They never seem to show any verifiable income or bank accounts, which is problematic as we have no gauge on their ability to cover their portion of the rent/utilities (unknown above)Most simply tell us not to worry as they have a voucher but when we start talking to their case worker(s) it is usually a different story.Does anyone have a different information form for S8 prospects (after they identify themselves as such) that requests things like: Name & phone # of case worker, # of bedrooms approved for, housing authority portion of rent, tenant portion of rent ?

Different companies use different terminology as this is a fairly unknown product.

Would the exact costs of some of the items included in the closing costs be unknown at the time of agreeing on the purchase price?

if the cops won't put him out then you can file eviction for the unknown/unauthorized occupant(s)