Point is intelligence based in Europe is still under legal control, NSA is not.

If there is big issue in future, you can summon European intelligence at court.

You're not wrong, but with Five Eyes, the OP's point still stands strong as ever.

https://en.wikipedia.org/wiki/Five_Eyes

In practice, there's little advantage to living in Europe, being spied on by the NSA who then sends that data to European intelligence agencies.

Considering that France recently voted a law to make retroactively legal existing practices, I'd say this legal control is most often a farce (and it's not like the new law introduces strong judicial supervision, either).

GCHQ's operation legality has been put under question more than once.

That hasn't happened in past does not mean it won't happen in future.

>Getting spied upon by GCHQ, the EU branch of the NSA, or by the mothership itself, makes little difference in practice.

It makes a difference depending on whether you're OK to be spied upon by a foreign power -- who might damage, take advantage and exploit your country.

Would Americans be OK to be spied by foreign powers while in the US?

And if yes, would the answer still be yes if the US wasn't the top dog, and the other country was, and could hurt them whenever they wanted?

Do Americans have a realistic expectation on not being spied upon by foreign powers over the internet? A cursory look at securelist.com reveals that multiple nation-states conduct campaigns against U.S. targets (and also reveals apparent U.S. gov campaigns against targets in other countries).

>Do Americans have a realistic expectation on not being spied upon by foreign powers over the internet?

Maybe or maybe not, but they do have a very realistic expectation of it having no consequences to them and the country at large. It's not like China (much less Russia) will do anything to the US (as a land and population). Other countries even less so.

Whereas lesser countries have seen the US (and Russia, especially as USSR and/or China) mess up with their internal politics, even topple legitimate governments and install friendly lackeys in power, assist one or the other party to get in power, help friendly governments prosecute their citizenry (especially during the Cold War), bully them to sign favorable trade and other deals (favorable not for them of course), or downright get to war with them.

I don't know a lot about international electronic communication surveillance, but wouldn't GCHQ be responsible to the United Kingdom, whereas Germany would spy through the BND and France would spy through the Brigade de Renseignement?