Salesforce archives certain activities, which cannot be viewed in Salesforce Lightning. To view these, you may export, or view them in Salesforce Classic. You can read more about this here.
For detail on the Salesforce criteria for archiving activities, please read here. Please note that this will apply to your imported notes as well, since we import those as events.
Please also note that while Salesforce Lightning includes the advanced Email feature, there are too many nuances with their current API for us to guarantee that importing Emails into this feature will work with your migration. Instead, we import Emails as Tasks, as this is how Salesforce Classic treats them.
For more information on the advanced email feature, please see here.