The pitfalls of a local only education aren't immediately visible. For the most part, if you work hard, get good grades and graduate from a local university, you'll land yourself a fair job afterwards. But when you first start your career, you often deal with menial tasks or have to do further professional training. This is especially so in Chinese companies, where it's more top-down and all important decisions are made by management. Everything could seem fine until well into your 30's, when you're mid-career.
If you were great in the local education system, you'll probably do well up to this point. That's when you might get involved in some strategic or creative thinking. By that age of course, you're well past your prime for learning to think outside the box. In some ways it doesn't matter, because many Hong Kongers are happy to work professional jobs which pay decent, but are not management or executive level.
We often compare local and international school kids whilst they're still teenagers, but that's really too early. It's better to see how they fare later on at mid-career level to see how they compare in the real world longer term. On the flip side, there'll be many international kids whose Chinese is so poor it affects their odds of landing a graduate job. If they can't get their foot in the door, they won't be around to compete at mid-career at all!