TL;DR : WC Variable product aren't imported corrrectly with each migration method
I'm having an issue when importing variable products.
I used some methods to import them: WooCommerce Import/Export CSV; Wordpress XML Import/Export, Plugins...
No stock and no variations imported for each variable product. (But all variation fields are filled on the export CSV)
Simple products are correctly imported
Solved !
I checked error logs, it's due to a 429 error from the old website during the migration.
The solution is to disable the host spam protection, themes and plugins from the old website.