Polish here, very much against adopting the euro until our standard of living and growth rate matches Germany (no at least not for next 10 years). Why? Because the disadvantages far out weight the benefits for developing countries.
The biggest issue is giving up one of the biggest instrument of control over the economy to a supra-national non-democratic organisation. Surely the monetary policy will follow what is best for the biggest economies (or at best the average) while local policy is way better tweaked towards local needs. The best example of this is money supply. The money supply ideally should match the economy growth rate +X so there is tiny inflation (and definitely no deflation). This growth rate is very different in "old EU" and "new EU" countries. So what happens? In time things get more expensive much faster in countries that grow faster while incomes stay the same. This is a huge negative and this is on top of price increases happening on "day 1" due to rounding up during conversion.
Historically the biggest benefit that was sold as something to outweigh this was a claim that "inflation will be low" and big inflation spikes are impossible. This came about from the short sighted view that all inflation stems from printing money and by giving up our control over it to somebody else we somehow "protect ourselves". This was proven wrong during covid when inflation was vastly different in let's say Latvia and Germany despite sharing a currency.
So what is the bottom line? Is euro all bad? No, it is very useful so we have a common currency in the euro zone that is not controlled from across the ocean. This is a huge benefit, but the same benefit is achieved by having it be a second currency like it is now in Poland rather than the only currency. (you can pay in euros in almost everywhere if you prefer as well as get it from cash machines etc)