Thanks for reaching out to the QuickBooks Community. I hope you're having a great day so far.
Because the systems subtly vary in how they handle accounting, it is not always possible to convert everything for all of your accounting period. Here's a list of some of the data that won't be converted:
Assembly builds
Budgets
Memorized transactions
Deleted transactions
Draft or unposted journals
Jobs that are not attached to transactions
Closed or partially closed estimates and purchase orders
Bank reconciliation history
File attachments
Invoice template and other templates
Terms list
Employees list
Payroll information, if any (employee details, pay calendars, tax information and pension, etc.)
Project details, if any (quotes and estimates, tasks and expenses, profitability, etc.)
Fixed assets management
Please note: It is important that you export reports from Xero and save them in a safe location even after the conversion is done for the tax or financial audits.
To know if Xero is still usable post-migration, I recommend consulting with Xero's Customer Care Team. This way, one of their agents can go over all the information you need before converting.
For more details about migrating, please check out the help guide I'm including below.