The Cuban crisis was about a hostile country (Soviet Union) that did not already have a land border or close proximity to the US, setting up shop close to the US with hostile intent. That was a *new* level of access to the US mainland that wasn’t there before.
Ukraine (western ally) already shares a land border with Russia. Taking a part of eastern Ukraine doesn’t change that fact. There is no greater or lesser threat toward Russia because of it (assuming there is hostile intent in the first place). The border just moves westward. The perceived threat toward Russia doesn’t change as a result of the land grab.