Many lenders offer flexible financing terms up to 85% in some instances for investment property transactions. 

"The agent assured me..." or "the property manager thought" or "the inspector missed" are all famous last words. 

You will have to first notify the tenants' case manager at their local housing authority who will likely want to dig into why you don't want to renew - in which case like others have mentioned, it's a good time to lawyer up and/or document every instance of them breaking lease guidelines. 

- What are the downsides for the landlord, compared to a traditional residential lease to a family for instance?

For instance, I'm great at finance and project management while I'm a work in progress for construction knowledge.

For instance, because you're closing on the 7th, all payments could be due on the 7th day of the month.

It may be state specific, for instance Texas prohibits naming the owner along with LLC I think???

Texas is in a challenging spot in terms of insurance.For instance, Foremost has not stopped insuring short-term rental properties overall but they may have in your area/Texas. 

But in any instance, negotiate an option to extend the maturity date.

I imagine we will see less and less of these subject-to transactions in the coming future as this method has been popularized by some famous proponents...As far as finding these deals, a sub-to transaction would most likely be found by trolling foreclosures and finding off-market deals like a wholesaler.