it's around as efficient as you get, but don't think that it's calm.
incidentally, there's an odd thing here - the train lines are pretty hectic. people don't cut that much, but they do cut some and you have to kind of defend your spot, people enter the trains before exiters are finished exiting, etc. it's not as bad as the youtube stuff i've seen from beijing, though i suspect that those are the worst of beijing....
but - and this is what's odd - people here are total sticklers for the bus queue. no one cuts, it's orderly and calm. there's even at times a secondary line for people who don't want to go back to the back of the main line (kinda like if they'd walked in from the wrong end of the hallway), and i've never seen them try to rush on before the main line had completely boarded.
buses are a legit option here.
except drowning in a disastrous ferry crash, of course.
not that this is a regular occurrence. just one of my random phobias. but you'd never get me to live on an island that relied on ferry service.
i actually don't even like the rush hour cross-harbour trains for this very reason. i'm going to die some way. i accept that. i just don't want it to involve water.
and fwiw - i've taken a fair number of non-rush-hour cross harbour trains, and they're usually not particularly crowded. red line and tung chung line. crowdedness of the HK to Central walk is pretty random - some sundays it's not worth queuing up for the conveyor belt, some it's easy to walk on it. i won't say that i like that change but for whatever reason - the lighting, the high ceilings - it's more pleasant than your average long transfer. it's certainly more pleasant than the change at kowloon tong, for instance.
If you are lazy and try to get on the mtr near the nearest point from the escalators, then the chances that most of the congestion lies at that point
On the other hand, if you are talking about going from, say, Causeway Bay to Kennedy Town across the top of the island, buses would be much slower than the MTR. And crowded. So - it all depends.
You mentioned Tung Chung. There are many buses that go through Tung Chung (the airport buses). I use the ones direct to the airport, but there are others (E-xxx) that go via Tung Chung, then on the freeway all the way. They go every 8-12 minutes depending on the route. Worth exploring and deciding if you prefer the experience or not. I do - i like to sit upstairs on the double decker and look at the view. Especially at sunset on the way home. But you may hate the experience and prefer the MTR - different people like different things.