I'm not a legal expert but I agree with the thrust of this article. The Catalan government seems to be disregarding the Spanish Constitution - and the basic principles of responsible democratic government - by claiming that 'people have the right to vote' on any question, whether or not the process is legal, and avoiding the question of whether the result would be used in a legally or morally acceptable fashion (i.e. a UDI). While it sounds good, if you think about it it's ludicrous.

Catalonia may have legitimate grievances, and the Catalonian people may want to secede, but this referendum is not a democratic or responsible way to deal with those issues. Both the Spanish and Catalonian governments have a responsibility to deal with the underlying issues in a civil and responsible manner. 

As an aside, my guess is that the bots are out in force in this comment section, as they often are where the stability of the EU is concerned.