Very true! And 3HK's indoor reception is particularly weak!
I've been using 3HK 4.5G/5G plan (全速) for more than 10 years. And I'm also using HK Mobile 365 days plan (限速 42Mbps). At several indoor locations (of different districts): 3HK 4.5G vs HK Mobile 4G -> 22Mbps vs 44Mbps .
Will switch to CSL in a few months.
ps. In addition, 3HK's 5G reception is nothing to be proud of