Serbia is historically aligned with Soviet/Russia ie on the 'Greek Orthodox' side of the Iron Curtain and so that's why it's going to take them that much longer to join the EU, if ever.
Edit: FYI [1]
[1] https://en.wikipedia.org/wiki/Russia%E2%80%93Serbia_relation...
But Poland would be done without the EU - imagine if 'New Mexico' were cut off form the US, it would be devastating. It's not just the transfer payments, it's the trade, quality standards, participation in so many groups, financial networks, zillions of Poles able to work abroad and remit payments, the Poles do not have a sophisticated diplomatic corps so they'd be starting from 'nothing' in trying to make international trade deals and nobody would care.
From a geo-strategic perspective, Putin & Co. may very well press hard to meddle in their affairs, trying to pull them into the Eurasian Economic Union aka the Neo Russian Empire.
So in almost every way, Poland wins tremendously from EU participation and that's probably unanimous from every perspective. Arguments between Duda and EU is huff and puff, partly it's just populism, partly it's negotiation, partly it's Duda trying to draw the line at where he believes power is etc. etc. but it's not existential - there is zero chance Poland is leaving the EU.
[1] https://www.pewresearch.org/global/2019/03/19/europeans-cred...