I remember an ATO telling me once, when doing my instrument rating, that the data from the METAR is older than 15 mins when you read it so thats why you couldnt use it to take 100ft off the minima.
Yeah, I understand that with QNH from a METAR or a TAF, you're not going to be able to take the 100ft off the minima, but the AIP suggests you can't use the METAR QNH
AT ALL. It lists 3 sources of QNH that need to be set prior to passing the IAF, and METAR QHN isn't one of them.
I'm curious as to why that's the case.