We are using DBSync to create invoices directly from a SF opportunity to an invoice in QB Online. It works really well. The receivable arrives with the invoice data related to the opportunity, which is great.
However, it's still a pain sending out the invoice to the customer as my email addresses are all in SF and we like to monitor all emails to customers through SF.
I have recently acquired a great deal of new data which has been upserted and cleaned in SF.