An ISA is a tax wrapper, not an investment — what matters is what sits inside it.
The wrapper and its contents
An ISA is a tax treatment applied to whatever is held inside it, not a product in itself. A cash ISA pays interest and so raises the question directly. A stocks and shares ISA raises no issue of its own — the question is entirely what the underlying holdings are, which returns to the screening above.
The same reasoning applies to any tax wrapper in any jurisdiction: assess the assets, not the label on the account.