Trusts and private companies are used for many families during their working and business lives. The issue arises when they retire. See the attached article drawn from a real case study which appears commonplace.
The key points are:
If you have private trusts or companies or any other sort of potential assessable income or asset where the values cannot be easily and readily determined then plan early and think ahead. The process for assessment by the government is lengthy ,invasive, and they look back at many years of history.