> Is this because of USA being a federal union? Before Ukraine declared independence, there were voices to make Ukraine a federal state, so that people in the West part of Ukraine can live their way of life and people living 1600 km (!) away in the East and Southern parts would be not much affected from that and vice versa.

You are falling for Russian propaganda about evil western-Ukrainian nazis attempting to enslave peaceful-Russian-speaking-peoples-of-Donbass-or-whatever who were just minding their own business ("way of life"). As a Russian-speaking Ukrainian neither do I want Putin to protect me (apparently by looting my apartment and raping my girlfriend or in whichever way he is trying to do it these days), nor do absolute majority of population of, say, Kharkiv, Odesa or Kherson.

> Voices for the unitary state were stronger because of stability of the state. Would be interesting to see some documentary "what if", whether a federal state would be more stable against pulling from the west (Europe, US) and the east (Russia).

As a Ukrainian I find that idea quite laughable. It is not really possible for a part of federal union (say a state of USA or a Swiss canton) to join NATO and for other part to "decide" to become a Russian-occupied quasi-state like Belarus. Same goes for a part of it joining EU while some other part decides it wants to be part of EAEU Customs Union. State's foreign affairs are still decided by some central government.

Also, you can research how great "deciding on their own way of life" works in Russian Federation. You could start with first and second Chechen Wars.