[pluma]

Scotland is in a different position because the UK already has varying degrees of constituency (e.g. compare crown dependencies, "countries" and so on). Over the past century the UK has effectively paved the way that enables votes like the one in Scotland in the first place (contrast this with Ireland a century ago).

But you are right: what matters isn't whether the vote is legitimate but whether the sovereignty of the seceding territory is (internationally) recognized. This is certainly not the case in the Ukraine, nor is it the case in Spain (Catalan) or Turkey (Kurdistan).