Firstly, you can use metal sockets with plastic conduit. The round stuff actually fits very neatly with proper screw on ends. Metal boxes have knock outs on all sides. Only reason to come from above would be moisture ingress, not really a big deal inside unless the garage is very leaky.
I have all of mine in plastic, I have fitted stuffing glands to all entry points to boxes.
If you have outside lighting, ensure the cable has a droop on it before it enters the wall, on the outside.
If you paint the walls white, use black conduit, its less likely to get hit as you will see it, I didn’t and have hit stuff a few times.