Regardless of the person who writes your will and the multi jurisdictional nature of your assets, I do think it is essential to read the FAQs on the Judiciary site, related to probate in Hong Kong. Much like life insurance, you need to consider the beneficiary and their abilities to get access to your estate.
For me personally, I'd go with a solicitor. If I had assets in multiple countries I'd put that into a "mo money mo problems" situation and pay for advice from a firm which has the ability to service your beneficiary in multiple countries.
In particular in HK these are the reasons why I'd go with a lawyer or firm with a long standing relationship in mind.
11.3 Why does the staff at the Probate Registry have to ask questions and request for documents?The Probate Jurisdiction is inquisitioral.
The Grant is an important legal document which usually authorises the grantee to deal with the estate without limitation. Hence, the Registrar is under a duty to make inquiries and see that they are answered satisfactorily.
These two lead me to believe that a solicitor will have to be retained for any non-trivial matters - death is a non-trivial matter as is making sure that the beneficiary has to deal with as little fuckery as possible from the legal establishment.
8.5 Can I apply through an agent?
Rule 4(2) of the Non-Contentious Probate Rules (Cap.10A) aims at preventing persons, who are not qualified solicitors, from providing legal services. The rule prohibits application through an agent, whether paid or unpaid. Further, the personal applicant may not be attended by any person acting or appearing to act as his adviser.
And the fact that the Law Society can search for a will, which might be lost or unrecoverable for whatever reason.
Not saying a will writer is or is not an option - legal advice should always be sought.