I went to one of these prestigious convent schools from nursury until I left HK at 18 so I know exactly how it is to be in such system. To be honest, there were homework (i didn't think it was a lot though) and frequent tests, a mid term + final exam. I didn't do very well academically, just managed to pass most subjects and the school encouraged me to develop my interests in sports, so I did have a great time and enjoyed school a lot. When my F5 public exam results came out, my scores were pretty good despite my poor grades at school, so I knew from that point that one benefit of going to these local prestigious schools is that you are likely to do much better than kids in average schools (academically). We shouldn't place that much focus at academic results but unfortunately this is how the HK education system works right now. For families who don't have the option of enroling their kids to international schools or overseas education, going to local prestigious primary and secondary school would just give them a better chance of getting in to local universities.
Marymount was one of those very good schools at my time and I heard it still is. If I had a girl, that would be one of my top choices.