What do you mean by "Amazon is not available in all EU countries". Do you mean like a country specific TLD? Because that is true, but order and delivery is not a problem from any EU country as far as I know.
It just comes down to suppliers, who aren't serving customers outside of select markets for whatever reason.
Aside. I've seen the exact same problems with our local Amazon "competitor". As soon as they became a platform marketplace I've started using them less and less because of the same quality/delivery issues.
Which is that even if it's legally a single market it's common that stores that deliver something to your door only sell their products in their own native country, or only within their local region.
They're not ordering from amazon.com (US), they're probably ordering from amazon.de with a thin layer of English translations + autotranslations.
I've used .de very often in the past, and when I stopped using Amazon, as said in reply to someone else https://news.ycombinator.com/item?id=31445473 , quality dropped (among other downsides) when they allowed third party sellers and those flood the product listing pages.
With those specific suppliers (and anything that isn't fulfilled by Amazon), I expect to be possible to have the issues you're referring to. But I wouldn't clump that together with Amazon not being available in some EU countries.
Yes, theoretically you're right, but in practice it's useless advice.