If you think it's a bond... you're wrong! Private mortgage banks are also subject to government rules and are transparently managed institutions.
If unexpected troubles arise and it becomes impossible to obtain a mortgage as the deadline approaches, getting a private mortgage for just a few months or one year can be a good solution. Most private mortgages can be set up as open and there is no penalty for paying off the mortgage at any time, so they often charge high interest rates.
When large corporations build buildings, they often use this type of mortgage and pay it off in one lump sum after completion and sale of the property.
It is possible to obtain this type of mortgage even without income, as long as you have a certain amount assets enough to down pay.