During the Yugoslavian wars, Croatia had more victims then Serbia. And their infrastructure went to shit too during the war. Of course, not as bad as Serbian infrastructure after Nato bombed you (and killed around 1500-2000 Serbian people, mostly soldiers) but still. Anyway, I agree with you that they would be ahead, you started (and lost) 4 wars after all, so it is normal to be significantly weakened after that. Anyway, Croatia was always ahead of Serbia on economics (as was Slovenia). I don't know why are you trying to argue that.
I never said that joining EU = all problems gone. It helps though a lot. How would have Greece survived without help (be it money given or loans given which nobody knows when/if will be repaid) from EU (especially Germany)? If they weren't part of EU no-onw would have cared to help them. If you manage badly your economics for a few decades then EU or not you're going to pay the consequences. But it is completely different business if you're in or out of it (unless you are Norway and have more money then you need). It is also the reason why your leaders are desperately trying to become part of EU (despite most of EU members, and all of significant ones bombed you) and you have to listen what Merkel says.
The same goes for Ukraine. They can decide to stay as they were, be Putin's luna park and face his wraith anytime they don't listen, suffer in process for being weak economicaly, or they can choose to become in the future an EU member and gradually improve their economics. Whatever they do, the choice should be theirs because they will have to live with that choice. If I was an Ukrainian, I know what I would have chosen.