Materials have to be stored somewhere , usually the last room to be done . 

They must have a great computer and internet and backup internet.

Search for land on internet How did you finance this deal?

Regarding downsides, or perhaps good barriers to entry, saturation is pretty strong, good help is quite hard to come by, inventory has been hovering around 25 or so homes for a while (most of which are not STR material), and much of the recent cabin listings have been grossly overpriced.

.$30/board should cover all labor and all materials...

This is pretty much how banks do things too - the idea is that not only do you have to afford your rent - but also your electric, your food, your car, your insurance, your gas, internet, etc.  

When I do, I use a written agreement that explains specifically what they will do, who pays for what, the materials to be used, a completion date, etc.

The Internet, social media, will repeat information. 

As you have experienced, trying to predict what will happen through the course of two years is very difficult - contractor integrity, materials costs, what the sale price will be, what the rental price will be. 

This includes construction, materials, labor, permits, and any other associated costs.Negotiate TI Allowance:Landlord and tenant negotiate the TI allowance amount based on the estimated total cost of improvements.