The BIG difference is that, at least for foreign domestic helpers, it is ILLEGAL in almost all circumstances to have them live out (there is a very narrow exception for helpers who have been working with the same employer, where were already living out before the law was changed).
While I'm sure you know plenty of people who do it, if your helper lives out, you are putting her livlihood at risk as she will almost certainly be deported if found out (and it isn't unheard of for other jealous helpers or nosy neighbors/ building staff to report live-out arrangements to the authorities). You are also risking your ability to hire another helper in the future if found out.
Since privacy is a big concern for many people, we have come to an arrangement with our helper where she can do whatever she lives between when she finishes work and 11pm, when I go to bed, whether or not in the house. We prefer her under normal circumstances to be home by 11pm as our dog barks and wakes me if I am asleep and she comes in after that. However, we give her both weekend days completely off and she is welcome to stay with friends at that time (sometimes she does, sometimes she doesn't).
You definitely need to be flexible to make a live-in arrangement work, and find the right helper, but many people make the arrangement work fine.
For local helpers, who can legally live out, I have actually never heard of one living in. But, it can be harder to find a local helper who speaks a language other than Cantonese and communication could, if you don't speak Cantonese, be a little difficult. But I know people who have part time local helpers who make it work just fine, with some patience.