It wasn’t the US, but not a EU country.
Basically the first issue was that the country in question has a withholding rule where 10% of the income automatically gets withheld by the home country of the company.
I then needed to obtain a certificate of tax residency from the company I was to be working with each month which then goes to HMRC.
The risk was that if there was a delay or difficulty in obtaining anything then it could cost me more tax or worse, fined. My experiences in 20 years of dealing with HR / Payroll is that things don’t move quickly.
The other issue for me was liability insurance. This is probably industry specific but as a design contractor working with very large tech companies, I’m required to have liability insurance to cover an enormous sum, I can’t remember exactly but I think it’s a billion pounds or something.
It was a grey area as to whether I was covered with this inter-country agreement so a risk not worth taking.
Again so much of this is unique to the individual, the industry and the companies involved so I think you’re doing the right thing by seeking an accountant/legal aide.
Sadly it’s just tricky to find a good accountant. One factor I think might be at play here is that some accountants have a flat fee for end-of-year small company accounts, so obviously if it looks like its going to be a lot of work, they might not be interested.
Sorry I can’t be of help on the GDPR thing. That’s a whole other mountain of legislation.