It is highly recommended to use our CSV importer if you're considering to import over 100k customers.
When you import a customer list from a native integration, they are added in chunks rather than all together. So the larger the list is, the more API calls will be made to the service that has your customer data stored. Sometimes this might cause issues and your customers will not be imported or only partially imported.
The most reliable solution is for you to import all your customers through a CSV file. If the list to very large, it might take some time until they're imported, but we will notify you by email when it's done.
Next, if you still want to keep your list up to date, you can go and connect the chosen integration and enable the auto-sync option. Your existing customers will not be imported again from the service you've connected to, while the new customers will be automatically added.