1.  ‘Sham’ – see para 7.8 of my book the ‘Contentious Trusts Handbook’.
2.  ‘Illusory trust’ – see para 7.9 of my book & use the search box at the top of the blog to search for – ‘Challenging the validity of an offshore trust as being an illusory trust.’
3.  ‘Jurisdiction’ – use the search box at the top of the blog to search for ‘Challenging the jurisdiction of an Offshore Trust.’
4.  ‘Failure to constitute’ – use the search box at the top of the blog to search for – ‘The offshore trust that never was!’
5.  ‘Validity’ – use the search box at the top of the blog to search for – ‘Can the validity of an offshore trust be challenged in the English court by HMRC seeking a determination of the applicable law?’
6.  ‘There is [also] a potential elephant-sized trap in conducting the mediation of an offshore trust dispute face to face in an onshore jurisdiction [which in my experience very few trust practitioners appear to be aware of /switched on to], as a revenue authority may challenge the tax residence of the trust; therefore, it may be necessary to obtain tax advice before agreeing to the venue. … [G]reat care should be taken when setting up the mediation of an international trust dispute.’ – See my article ‘Mediating Probate and Trust Disputes – Process Challenges and Tools – Part 1.’ Published by Oxford University Press in ‘Trusts & Trustees’ worldwide in the journal in February 2023.
I can also think of several other technical ways in which a revenue authority could attack an offshore trust – but nobody is asking!