USPS tracking is very accurate in most cases.
However there can be some delays in the USPS tracking updates.
When tracking a USPS package, the tracking only updates once they are scanned so it can take awhile for the USPS tracking to update and show where it's at.
Once the package leaves a post office, the package will get a scan indicating it has left the post office, then once it gets to a sorting facility, the package will then be scanned again and say it's at a sorting facility.
Once the package gets ready to leave the USPS sorting facility again it will say that the package has departed the sorting facility and will update to say that it's in transit to the next facility.
After your package has reached a sorting facility that is near to your address, then it will say arrived at a USPS destination facility.
Then once the package gets to your local post office and is ready to be delivered it will say arrived at post office and then out for delivery.
Finally once the package has been delivered, the USPS tracking will say out for delivery.
It can take 3 to 7 days for some packages to reach your address depending on how far away the package is.