The data will be accurate within what FB reports, but we can't guarantee FB reporting accuracy overall -- iOS14 had a huge impact on FB Ads reporting, and made things a lot less trustworthy.
One thing worth exploring is the performance of FB on a last-click basis using source/medium values. You can find this data in a couple places including the Orders and Customers deep dives using the
source_medium filters (to scope the whole page to specific source/medium(s) ), or by looking at the source/medium-specific visualizations.
Facebook Platform Attribution Discrepancies
The number reported in Marketing Performance report for Facebook are strictly a 7 Day Click Attribution. This can often lead to discrepancies because someone need only click the ad in order to be attributed, however they could've bounced complete. They could then see an ad on another platform and bought and this action will attribute the transaction to that source medium.