It has to be possible to set fields of UserOptions via REST.
At the moment it is possible to use a REST call to open a request for an offering without filling in the UserOptions, even if they are marked as mandatory.
In my opinion this is a major failure.
However, in my opinion it is not enough to prevent this, but it should also be possible to order an offering via REST, just like via the portal. This includes the specification of UserOptions.