The export to desktop (US Only) provides you with a file that must be opened in desktop within 10 days, and the file has known problems, Some things do not export, inventory exports as service items with no history, payroll if you have it is exported as regular checks, these and other known problems have existed for over 9 years
see the limitations:
https://quickbooks.intuit.com/learn-support/en-us/manage-data/export-limitations-quickbooks-online-t...
Print out the QBO P&L, balance sheet, inventory valuation summary (if you have inventory), and sales tax liability as a minimum, compare the QBO reports to the same reports in desktop after you have a desktop company file - they must match, if not you have to find and correct the problems, intuit will not help
IMO if at all possible, it would be better to just re-enter all transactions in desktop
