A true union has a national pay scale, a national contract, and national seniority. It also has high barriers to entry (ask any "journeyman" plumber).
There are no true unions in aviation in this country. Teamsters Local 747 is the closest thing we have to a real union for pilots. I say that, because even though 747 is the most anemic branch of the IBT, they still know how to fight and win, and have the backing of hundreds of thousands of members nationwide. When Hoffa speaks, people listen.
ALPA is not a union, by design. It's an ASSOCIATION of independent MECs. This setup was intentional, because the founders wanted each group to have flexibility, and also because unions at the time had a huge stigma and they wanted to appear above that. This setup makes ALPA weak, because there is no TRUE national agenda. This is best evidenced by the Age 60 reversal. ALPA's national policy changes with the wind, depending on which group (who pays the most dues, and thus has the most power) speaks the loudest.
The result is infighting, backbiting, and a general lack of unity you don't see in trade unions.