Look at RATE of growth (GDP, employment, safety, etc.) since immigration started getting bad in places like the UK - compare it's growth directly to the UK, or even the entire EU
* developed countries obviously develop slower than developing ones. Is easier to improve if your economy is shit especially if you join a union of more advanced countries.
* polish immigration actually skyrocketed recently since Russia invaded Ukraine. It didn't harm employment, safety, growth rate or anything else yet.