For the US Person with a non-US residence, the topic greatly depends on each individuals own financial and personal circumstances. For the US, ISA's are just various types of taxable accounts with different names, and depending on the type of account they require various methods of reporting. Greater ISA profits generates more income taxed by the US.
Thanks to effective lobbying, Congress has favoured the funds going into a US account for US Persons. Taxation is well defined. Simple, but for the fact that the country of residence may declare the gains made in the US to be taxable income in that country of residence.
As long as they stay in an ISA, the gains are tax free in the UK. Are there any 'forever' tax free accounts in the US?
In the end, it comes down to whether or not you'll actually pay tax on the income (in the US, or the foreign country, or because of your personal situation or the type of investment/reporting, neither), and if so, which tax rate is the more favourable.
We're back to the issue of the US taxing on a citizenship basis. That's a separate thread. For this thread, the name 'ISA' has no bearing, and it's up to each individual to review their own circumstances regarding taxation of the accounts. Regardless of circumstances, the annoyance remains the currency fluctuations. As long as we're taxed on citizenship, that won't go away.