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.
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.
One of the dangers of opening a country's border is that it affects local industries negatively. If imported goods are cheaper people will buy more foreign goods and fewer goods produced by indigenous industries which makes them lose market share. When they don't make a profit it will lead to a decline or fall of the industries thereby encouraging unemployment. Donald Trump once shut the American borders against Chinese goods to boast the local industry of the US. Just recently some countries in Europe had to control their borders to reduce the inflow of cheap Ukrainian grains because the local farmer needs to be protected from losses.
But it could lead to the transfer of technology and ideas. People can migrate to a country with skills and technology. When goods and services freely flow in and out of different countries there is always a transfer of investment, financing, skills, and technology. The best carpenters in my country are from a neighboring nation and they have gradually transferred their skills to the local carpenters.