I've generally found they're all correct. Sometimes, I run into situations where a particular exit is closed because it's after-hours (some stations close certain exits when it's late), and Google Maps doesn't realize this, but I haven't noticed any exits being outright wrong.
I think this is probably a local issue: Google Maps is better in some places than others. Google has a huge office in Tokyo, in Shibuya overlooking Shibuya Crossing, so Tokyo is probably one of the best-supported cities in GMaps for this reason. Other cities might not have such accurate information.
Stations undergoing maintenance or upgrading may have certain exits closed or even temporary platforms that change as the project goes on. Here in Chicago, the Red Line is undergoing a major rebuild that started over 2 years ago and will not be completed until 2025. Temporary stations have been set up at multiple stops, with different entrances that have changed as the project has gone on [1].
It may have only changed a few times over the last 2 years, but it's certainly a rude awakening when you're running to catch the train and you find out your stop is now a block away from where you thought it was.
