Mass migration can be a good development because it creates an influx in the workforce and a diversity of thoughts and ideas. However, there are drawbacks. If the developed country is already having issues with overpopulation or high unemployment, then mass migration can only comound these factors and make it harder for everyone.
I believe that migration makes diversity and diversity adds to a workforce in that it brings different people with different attitudes and thoughts and solutions to the table. It also provides people to do difficult and tiresome labor that many people in developed countries will not do for the low amount of pay, but they are services that we all use, like the consumption of fruits and vegetables.