The MTR “train trip planner” recommends that for early and late departures from East Rail stations to the airport you should go via Hung Hom, Nam Cheong & Tsing Yi. This seems like bad advice, because it requires you to use two of the worst interchanges on the MTR:
At Nam Cheong, you can stroll across from a northbound West Rail service to a southbound Tung Chung line train (towards Central), but if you want to switch to the northbound service (to get to or from the airport) it takes five minutes to go down, across and then up again.
At Tsing Yi, the Tung Chung line and Airport Express tracks run parallel to each other (picture from here). But here there’s not even one cross-platform interchange - you need to go all the way down to the ground level, cross under the tracks and then go all the way back up again. Which probably takes 5 minutes. Would it really have been so difficult to provide a simpler and quicker cross-platform interchange?
Of course there are plenty of very good interchanges. To switch between the Kwun Tong line and the Tsuen Wan line you simply have to walk across the platform at either Mong Kok (same direction) or Prince Edward (opposite directions). But you should ignore this misinformation.
Or there’s Yau Tong, North Point or Lai King. Here, the most common interchanges are cross-platform, with a short walk up or down if you are going in the other direction.
Admiralty (plan) is good for the Tsuen Wan line to Island Line, but now there’s also the South Island Line, which is 3 or 4 levels down (past the so-called Sha Tin to Central link platforms, opening not very soon).
Others are not quite so easy. You’ll have a long walk at Quarry Bay (or a long wait for a lift), but North Point is an easier alternative for most journeys.
Kowloon Tong is a very busy interchange with a bit of a walk from one line to the other, and the layout is somewhat confusing. It was designed as an interchange station - the Kowloon Canton Railway (KCR - now known as MTR East Rail) started operating in 1910, Kowloon Tong station KCR station only opened in 1982, a couple of years after the MTR station - but the physical layout of the two lines makes it difficult to do any more.
As an alternative you can change to West Rail at Hung Hom (not bad - cross-platform interchanges available, if you don’t mind waiting sometimes) and then again at Tsim Sha Tsui but that’s really two separate stations for East Rail and the Tsuen Wan Line (to be fair, it’s shown quite clearly on MTR maps).
The normal route from East Rail stations to the airport is via Kowloon Tong, Prince Edward, Lai King & Tsing Yi. That’s one one extra change, but two of them are simple cross-platform interchanges. It’s also clearly a more direct route.