And possibly Trojans that grab all your phone's data nd send it to some server so someone can steal your identity. Stick with the Play Store, Amazon App Store or other known-save sources.
If the app or game isn't available on them it's either because it's too new (it takes a few days for them to check it out), it won't run on your device (because it needs more RAM than you have, it needs hardware your device dosn't have, the developer set it up wrong and it will run, but it says it won't ... many things) or it doesn't meet their guidelines (there was a rape game once - Google had it up for download for a very short time [probably done automatically], until someone pointed it out to them and they pulled it).