Most of Williamson's games overseas came in his first four years when he was bang average even at home, he was 19-23 at the time. Since 2014, he has averaged 72 at home but seldom gotten opportunities overseas, number of Tests he got in the big three and South Africa.
India 3
South Africa 2
Australia 5
England 6
The number in India and South Africa is too low for anything real to be extrapolated, two matches where he didn't even do badly and three matches split over 10 years say nothing. he has done well in Australia and below par in England (does have a hundred and the Southampton WTC final performance).
Comparatively, Smith in that timeframe has gotten 8 Tests in India, 16 Tests in England, 6 Tests in South Africa. Joe Root has 10 Tests in Australia, 14 Tests in India, 8 Tests in South Africa. Virat Kohli has 17 Tests in England, 14 Tests in Australia, 9 Tests in South Africa.
Other than those four, he has scored big runs in UAE and Pakistan where he has played 8 Tests and made two great hundreds on turners, in West Indies he got three Tests and won a series against a respectable attack. His home record is magnificent, an average of 72 over the last 11 years, winning games left and right and centre, he plays three in New Zealand and that's a difficult job and he has chased down great totals by himself. Green Mambas, fast wickets or even disintegrating late wickets he has made heaps of runs in New Zealand. All that doesn't go away because of one off bad games in India here and there.