Are you going to check-in in HK or Kowloon station?

I always take the 1:00pm HKG-SFO flight home, so I check-in at the Kowloon station in the morning (between 9:00-10:00am), UA desk is manned at that time, so I am sure it is no problem to check in and drop luggage.

They always ask me if I am heading right to the airport (which I am), but I once asked them why? They said that there is a cut off time, the last train they suggest you take in order to make it to the flight on time. So I am sure you can check-in drop your bags and ask them what the last train is that you can take in the evening to make your 8:00pm flight.

To answer your topic questions, yes, the airport express check-in at the stations is one of the most efficient system in the world (IMHO). I have never had an issue with it in the dozen times I have used it.

Hope that helps!