Unions also require the right structural conditions.

For example waste management generally has very strong unions in the developed world because 1) capital intensity 2) operations immediately seize on labour withdrawal 3) consequences of strikes are in a golden zone of painful but not existential so are tolerated 4) extract a relatively small proportion of local gov funds, so don’t create that much powerful opposition 5) labour force is locally concentrated and highly socialised.

Those pillars of power don’t rely much on any particular legal or political system.