just as "subsidiarity" is more than a slogan about "small government", the writing and thought of Leo XIII on the social question and the social order is not reducible to "unionism, as presently defended and advocated for in early 21st century America, is to be supported by faithful, thoughtful Catholics." It's not that unions were once necessary, but now they are not. It's that unionism is to be supported by faithful, thoughtful Catholics when it is consistent with, and actually carrying out, Catholic Social Doctrine, and not (or, at least, not necessarily) when it is not. To resist overreach and bad-acting by unions is, well, to resist overreach and bad-acting; it's not to stomp on Rerum novarum.
In my view, it is vital to keep in mind, as we try to think with Christ and the Church -- and not with either the Chamber of Commerce or the Democratic Party -- about union-related policy, to take into account (to the extent we can) the costs and benefits of proposals and practices, and to look at what unions are, and are not, actually doing with the power they have, and not merely to wield a "the Church teaches that unions are good" stamp. In fact, unions and unionism are sometimes bad (just as religious freedom -- which is good -- is sometimes abused).
For example: In the United States, teachers unions are, on balance, definitely not good. They have, historically, been a powerful force for anti-Catholicism and the obstruction of reforms, including reforms that the Church clearly teaches are morally required. It is a grave injustice to require parents who want their children to be educated in (reasonably regulated and reasonably well performing) Catholic schools to pay twice (that is, to deny public funding to those parents). Legislatures should not extend special powers to teachers unions, and they should oppose them to the extent it is necessary to re-orient education-related spending and policy in the best interests of children (and in a way that advances religious freedom and pluralism) and not of public employees who work in government-run schools. Another point: It is not good for unions to use workers’ contributions to support political causes – say, abortion rights – that are not relevant to the association’s purpose and mission.
Go read the whole thing. It's a very thoughtful analysis.