The quarantine and testing of incoming residents has evolved since it was started mid-March. The best solution in terms or infection risk for the local population is to quarantine everyone in dedicated centers, while sending positive cases to hospital. But these centers got quickly saturated. They are also used for contact cases of new positives in HK. At peak periods, the huge number of returnees from Europe and USA made it impossible. Some were released from the center after ten days and completed their remaining quarantine at home. But there has been many examples of people flouting the rules. In SIngapore, you get sent to jail or your residency revoked if you cheat, but HK is much more lenient. With the 100 thousands of returning students, quarantine at home was the only feasible solution.
There has been many reports of life in these dedicated centers (including from westerners who are contact cases). Frankly, conditions appeared quite adequate even if it is extremely unpleasant to be quarantined for 14 days. We now see HK families who had sent their kids home when situation looked bad in China/HK and return them now that the situation is bad in Europe and much safer in HK. Double national/PR make a personal choice and I find it surprising when they complain that food in those dedicated centers are not to their tastes.
My guess is that HK makes a decision on who to send to these scarce centers based on many parameters including the health risk of the origin country, the typical behavior of those nationals, their living conditions in HK, etc... We know that some countries do not have extensive testing capability and that the number of reported cases is highly unreliable.Nevertheless, reported cases have been rising rapidly in the past couple of weeks in countries like India and Pakistan. It would not be surprising if arrivals from these countries be sent to dedicated centers until they fill up (much fewer contact cases there).