Actually Hang Seng’s service is well known among HK locals to be far better than HSBC.
They are part of the same group but they are still separate companies.
I was very happy with them when I was living in HK, but I switched to HSBC after I left HK because HSBC just can’t be beaten on global coverage.
The other disadvantage of Hang Seng is that they are much less capable with English language service. It’s a much more “local” bank.