The biggest difference for me was not coverage, it was how much easier life felt when something broke. Once you are fully settled, 알뜰폰 is usually fine for basic data and calls, but the moment you hit a 본인인증 issue or need a weird account update, the big carriers still tend to be less painful. If someone travels outside Seoul a lot or is still fixing bank and app verification, I would save the switch until after that admin season is over.