It is more than assumptions.
None if those measuring the numbers can agree on how they should be measured.
Immigration from outside the EU carries quite a large cost to the government. Recent arrivals from the EU shows a surplus. But as they get older and still living here it shows it changes quite a lot.
Or you can look at how the numbers are worked out altogether. Someone comes to live in the UK. They pay more in tax than they take out of the system. But when they have children they are classed as British. So the cost of hospital care and then education is put to the side of British nationals. This keeps the nett higher for arrivals. But some count the children they have as 50% cost to both sides.
So it is down to how you want to manipulate the numbers. Non EU immigrants are said to pay less into the system than they take out. EU immigrants are said to pay more in than they take out. But when split to different countries that they came from some have a massive surplus but others take out much more than they put in.
This is the least biased I could find. It explains it quite well.
How immigrants affect public finances