At least the USB-C ones I purchase are not flush - I never found those to be reliable. It's a male prong that looks pretty much identical in wiring to the male prong on the phone, that connects to a female one plugged into the socket. That plus a bit of a collar to help hold it in place. So I don't see why there would be any difference in grounding, it's the same connection...

(that plus the comments from the more knowledgeable person below)

Eventually they start wearing out, and I just replace them. I've had no issues with high voltages (45W+ charging on phone and steamdeck) and with peripherals (hub for example).