Gains you'll get in Europe/UK: - much safer, more stable society for raising a family; - lots of history and culture (if you like that, I do); - much better public education, public transportation and public health systems; - much better work/life balance (35 hour work week and 6 weeks vacation); - a broadening of your perspective of life (Americans tend to be rather insular);
Sacrifices of leaving the US: - massive pay cut and much, much higher taxes; This means your net pay will be significantly lower than the US West coast. I am taking a 35% reduction in income; - if you work for a big tech company, their centre of gravity will be the West coast and so you may miss out on some project/career opportunities (this varies from team to team of course);
So, as you can see, from my perspective, having lived on both sides of the pond for decades, the only sacrifices of leaving the US are money related (I don't have any family in the US, though).
My decision to leave the US after 13 years is complex and, as a family man, is heavily motivated by where I think I can offer my daughter the best start in life. We want her to have a quality, classical education in a safe environment. For us, that means returning to Scotland and sacrificing the money of the West coast.
In addition, I have accrued enough wealth over my 26 year career to be financially secure (not rich, though) e.g., I own, with no mortgage, properties in the US and in Scotland. So, being older makes it slightly easier to sacrifice the massive (overpaid?) salaries of the West coast. If I were a young, single twenty-something again, perhaps I would optimise for the money. It's a tough decision.
A side note to counter the pay cut: one tech area where the UK is more advanced than the US is finance. There are a lot of interesting FinTech start-ups. Also, working in the City of London typically pays enormously well (better, typically, than a West coast big tech salary + stock package - for people my age, at least). But it's a very high pressured, high stakes environment compared to the campus life of a big tech company.