You go "outside" on the train platform at EWR as you're on the platform, and it is not enclosed, though there are enclosed waiting rooms on the platform. The same trains that go to Newark Penn (NWK) also go to NY Penn (NYP). Both of those are north of EWR, which means that you'll pass through NWK on the way to NYP. Based on that, you may as well just wait at NWK. It isn't unsafe for waiting at. That being said, the platforms are outside (though covered from rain) while at NYP you wait upstairs inside the terminal which will be immune from the weather.

You'll probably make the 8:22p train if your flight lands at 7p (or shortly thereafter). There aren't a lot of International flights arriving at that time, so the lines should be short. I know there is a CO flight in from Aruba around that time, but passengers pre-clear at AUA. Are you on that flight or a different international destination? If you pre-clear that will help significantly.

S.