The problem here is only that users shouldn't be making a username with the email IDs as the members will always be shown with the username in the URL when checking their profile and when the username is set to an email ID then it just can't be changed.

Also, BuddyBoss doesn’t make the email address the username - but this is what membership plugins are doing during signup.

This feature/option is already added to our Roadmap page. You can view the card on the roadmap page and leave a comment about how important this feature is to you and your site.