Here is a more detailed explanation: https://privsec.dev/posts/android/f-droid-security-issues/

Accrescent is a new appstore that fixes all these issues but its still in alpha stage and has 11 apps right now.

I replaced fdroid with Obtainium that pulls apks from github,gitlab,fdroid,etc and it has support for auto updates. It’s a little better than Fdroid but still has its own issues.

Why Fdroid is not secure:

1. Hosts an outdated APK client.
2. Utilizes an obsolete installation method.
3. Does not take advantage of modern appstore features.
4. Has no moderation.
5. Has no old app deletion.
6. Has an arbitrary FOSS only rule.
7. Does all building and signing themselves.

The Google Play Store is more secure.