O, you are asking how I know locals are harder working and more reliable? Yes I know this from the many people I know that choose to obide by the law and employ locas that they get from the labout department and other such institutions.
From your comment it seems you disagree, I take it you have had negative experinces with local domestics.
Nope. I am neither advocating that people break the law by hiring illegal workers, nor am I complaining about bad experiences with local workers. I just picked up on your statement, which can easily be read to mean that foreign domestic helpers would as a rule be less reliable, work less hard and are more expensive.
I found that statement somewhat offensive, but felt I should ask what you what you based your statement on, before responding...
There is of course also quite a large contingent of foreign men and women in Hong Kong, who are available as domestic helpers/drivers/gardeners/assistants and the like, who are perfectly within their legal rights to work here, on account of their visa status (either because they are already PR's, or have a Dependant Visa).
It's a common misconception that all part-time foreign domestic helpers are illegal, which is obviously not always the case. Given the level of English that I have experienced with local domestic helpers, this group may be an option for people who have yet to master Cantonese...