The accountholder has control over the value of a cash ISA,...
And therein lies the rub. Have a close read of Notice 97-19:
"
Determination of net worth. For purposes of the net worth test, an individual is considered to own any interest in property that would be taxable as a gift under Chapter 12 of Subtitle B of the Code if the individual were a citizen or resident of the United States who transferred the interest immediately prior to expatriation........
An interest in property includes money or other property, regardless of whether it produces any income or gain."
It says nothing about
control or the ability to transfer, only an interest. A pension is a monetary investment ("interest in property includes money") which the individual has an interest in, and that
could include the UK State Pension. It almost certainly includes many types of personal pensions. The same goes for a cash ISA (the vehicle itself) which cannot be "transferred immediately prior to expatriation", although equally the individual does have an interest in it.
And, that's just my opinion. I admit to playing Devils' advocate here, but there is "something" involved which is beyond my very limited knowledge.
I tend to think this, or something similar, is the "gotcha". The fact it is very vague is the reason CPAs, both in the States and for their counterparts abroad, will not go without food on the table. We're dealing with "foreign pensions", a term which seems to be left intentionally extremely vague in any IRS communication. This is where the CPAs earn their money, and know the ins and outs of what stance will survive in a "US tax court". I don't know what precisely is included in "the net worth test", but a qualified international CPA should know (CPAs practicing exclusively in the States could be dangerous in this situation).
Vagueness is the tax advisors friend. Unfortunately for us basic unskilled punters, it's the enemy preventing us from doing it ourselves. Anyway, a competent advisor will find a way for most to avoid the exit tax, even if it's advising to "gift" funds of up to $5Mil. in order to be below the $2Mil. threshold.