Brexit is bad because it’s a shitshow that was never seriously believed would happen let alone be properly planned. It will leave the UK in an objectively worse position in terms of power, economy and politics.
Catalan independence is bad because it won’t improve anything, it’s a pride of culture movement that will likely negatively affect the population. The EU stated that it wouldn’t accept a Catalan state as a member, so the economy and movement would be heavily restricted.
HK independence is good because the Chinese government and state apparatus is evil. Having your home and culture consumed and extinguished is not something anyone would wish for. Maintaining independence is objectively better than being controlled by China.