As a reasonably successful immigrant from Eastern Europe, mostly not true in my experience. You do have some people, ahem roma/gypsies ahem, that abuse the hell out of social benefits for example, getting them from multiple countries at the same time, etc, but for normal people, the reason they succeed is the fresh start and being an outsider in my opinion.

For older immigrants it's an opportunity for a fresh start that kicks them out of the daily monotony and puts them in the sink or swim of early adult life again.

For younger immigrants, we're mostly the top X% of talent coming from mostly well off families. Couple that with needing to be at least better than the average to compensate for your immigrant background and we either do very well, or go back to where we came from... After all with the experience of being mediocre in the west you can go back and sell yourself as a top achiever with experience in the country where the company headquarters/clients are.