APK ("android package") is the standard android app installer format. So the question in the title doesn't really make sense: it's like asking whether .exe is good for Windows. If you install only from the Play Store you are still downloading apks, it just isn't putting them where you can access them (without root).
I'm guessing that this is yet another thread from Kurt, since he seems to be asking essentially the same question in many different threads at the moment. So the answer remains the same: install from sites that check all of their apps, don't host cracked apps, don't host downloads from any old user who wants, and you are reasonably safe (nowhere is absolutely safe: both Google and Apple app stores have hosted infected apps before now, and doubtless will again). Install from sites that allow anyone to upload and share apps, sites that offer paid apps for free (i.e. are by definition engaging in criminal activity), and you should not be surprised if you find yourself infected.
Sites? I've already said that apkmirror is trustworthy. Many people say apkpure is OK, and they claim to verify all of their apps. I'd personally not go near aptoide, as they definitely do host unverified apps and have included malware and pirated apps, so don't have my trust (it doesn't matter that they have a section for verified apps, if their site allows such stuff at all they do not have my trust).
