Border opening can either be harmful or be helpful to a country, it works vice versa. It can be harmful in the sense that bad goods and services can be brought in,weapons can be smuggled in, illegal migration of non citizens can have quick access to a country when the border officials are corrupt.
The bad side is they'll never know the immigrants/refugees if they've got bad intentions or bad habits so it will eventually out when they are already there. Like this news in France, an immigrant attacked children.
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https://www.foxnews.com/world/syrian-refugee-injures-4-children-adult-knife-attack-french-alps-annecy-report)
It can also be helpful due to the price of goods and services because there will be competition in the market place there by making goods cheaper, it also strengthen international relationship.
Yeah, that's the good side of it. There will be more people working and there will be more taxation for them as it will contribute to the economy of the country where they'll go. Thus, it should be weighed properly if a country really needs them or not. Most citizens don't want to see immigrants in their country and that's one reality.