Apkpure and most of such websites such as apk-dl and apkmirror, simply download the apps from the Play Store and then upload the APKs on their websites. So, they are safe. But there are cracked or mod files of many apps, especially games, available on these websites. Now that these mods are created and customized by the websites, they may not be completely safe.
However, I do download a lot of APKs from these websites, especially the mod files and I haven’t faced any trouble while using them. They get installed and executed without any issues.