Followers & Unfollowers is one of the most downloaded Instagram tracker apps on mobile. The reviews are decent, the interface is clean, and it does surface the data you want. But it gets that data by logging into Instagram on your behalf. You hand over your credentials, and the app acts as you. That is not a design quirk. That is the only way it can work.
Instagram's terms of service prohibit third-party apps from storing or using account passwords. Accounts caught doing this get flagged. Sometimes they get suspended. The risk is real and well-documented across creator forums and subreddits. You may never have a problem, or you may lose an account you spent years building.
WhoUnfollowed takes a different route entirely. You request your data directly from Instagram through their official "Download Your Information" settings page, which is a right guaranteed under GDPR Article 20. Instagram prepares a ZIP file and emails it to you, usually within a few hours. You upload that ZIP to WhoUnfollowed. The app reads it in your browser, shows you your non-followers and unfollowers across snapshots, and closes. Nothing is transmitted to a server. The parsing code is open-source. There is no account to create, no password to share, and no ongoing access to your Instagram profile.
The analysis you get is identical. The exposure is not.