Modded apps are often unstable. They might crash frequently, fail to load, or lack features present in the official updates. Since you aren't downloading from the Play Store, you have to manually update the app every time a new version comes out, usually by revisiting the risky site.
Some websites sell shared login credentials for streaming or productivity apps for less than $5 a month. This is a gray area but generally safer than installing a malicious APK. mod4apkcom free