From an economic perspective, it's mostly positive. Raising a child is expensive, and those costs go on for about 20 years before you have a person that's economically productive. Most Immigrants are adults and can join the workforce immediately. The economic costs of their childhood was paid by the country they came from. It's a negative for the country they came from, this is refereed to as a "brain drain." But for their new country, it's like a tax paying worker just appeared out of nowhere.

As for the economic negatives, the big one is housing. Too much immigration all at once can result in a shortage of housing. It can also put stress on public services and infrastructure. Businesses may not have the capacity to serve a larger population. These things can adapt of course, but you can't instantly build a house and you can't instantly expand public services, etc. So you might want to limit immigration so an area can adapt to all of the various economic needs of a larger population. An immigrant will work and pay taxes and contribute to the local economy, so long term it's all positives, but there can be a lot of short term problems if a population grows to rapidly.

As for social... well I'm not really much of a sociologist, but just from I can see, people who already live in an area might be uncomfortable being around people of a different culture. Might say crazy things like "They're eating the dogs!" Yeah that's crazy, but it is a problem. Not caused by the immigrants themselves, but it's a problem that does happen when there's immigration.

But there's social benefits. Can learn from a new culture. May get some new options for restaurants to go to.

Generally the young will enjoy more social benefit (going out to the different restaurants and learning about different cultures), but the older people will tend to be uncomfortable with it. But that's just the tendency.

So overall I'd say you do need limits on immigration to mitigate the short term issues, but it's all positives in the long term.