For example, the wide majority that is needed for constitutional changes would require agreement at the federal level even beyond parties that are part of the government. Also, some constitutions have provisions that are explicitly listed as un-amendable.

In the case of Catalunya there are other issues. For example, admission to the EU of Catalunya would probably be vetoed by Spain and by almost any other country that has regions with strong separatist movements (e.g. Spain would have vetoed the admission of Scotland, had the Scottish referendum passed). This would be a big unknown for Catalan economy.