All correct about isa's, there's no catch other than the limit on contribution, which is admittedly fairly generous. Think of it like a Roth ira but with no penalty on withdrawals ever.
The uk taxes anyone resident in the country, citizen or immigrant, on their worldwide income. Anyone not resident in the country is taxed by the uk only on any UK sourced income. So if I lived abroad but had earnings from a uk based property or company, I would pay uk tax on that income, buy not anything earned abroad. Most counties seem to use something along those lines - only the USA and I believe Eritrea tax citizens on their worldwide income regardless of residency.
US citizens should not bother with an isa - not only will you still be taxed by the irs who completely ignore the isa wrapper, but any funds even index trackers you put in there would come under the PFICS regime which means massive form filling and penal tax rates in the USA. The only things that would not be hit by this are individual stocks or cash.
As to SheLivesTheDream's comment I agree in general, but would still always max my isa allowance before investing outside of an isa. I'm old enough to have watched tax breaks come and go; the £5k dividend tax allowance was a gesture to make the introduction of tax on dividends for basic rate payers more palatable. As such I would be unsurprised to see it quietly dropped in a few years - or more likely just not increased in line with inflation until it becomes pointless.
But overall yes, I've said it before that, the uk tax regime right now may be one of the better in the world right now to be FIRE in. Of course downsides include the famous weather, the high cost of living especially in London and the south East (of course if you're fi you don't need to be there for jobs) and the constant whining about immigration.