So yeah, more restricted buffer state than in the Cold War, which says something in itself...Being in the EU is not incompatible with being a buffer state. Well, not so long as you're not in NATO, at least.
And Yeah, Russia didn't want Ukraine to sign the Association Agreement with the EU because it made Eurasian Union membership impossible for Ukraine.
Functionally, that would be closer to what Soviet satellite states were during Cold War.
And those weren't rich.
I think the map you've posted may be one reason.And interestingly enough, the EU presented itself rather poorly in not even offering Ukraine any roadmap to EU membership, even if actual membership would be 30+ years away at that point in time (2013-2014). I don't see why exactly Poles and Romanians are more deserving of EU membership than Ukrainians are, to be honest.
There is also an interesting parallel with the state of Turkey, which is under an association agreement with the EU and formerly EFTA since the 60's, with no EU membership in sight, so it could be worse.
On the other hand, that in itself has certain economic and legal benefits over not having such a treaty.