Which trade union were you in at Armstrong and what exactly did they achieve for you?
I joined the IEEE, AES, etc.
TBH Armstrong were a pretty good employer and treated their line/repair/etc staff quite well. But I doubt that was true for many other businesses at that time.
Earlier, I had (briefly) a job at one of the old Defence companies. Nominally worked in a hangar with hundreds of other people. There I did join a union, but I can't recall which one. After a month or two I got fed up with it, so left. They had no work for me to do as they contracted projects on the basis of X people working Y hours for a Government contract, and inflated those numbers. Hence wanted many NOT to do anything. Boondoggle.
When an academic I joined the Union. They actually saved my job on one occasion, so it was a very good idea. They also negotiated good pay and benefits during that period. Alas, since then the pay and conditions at Unis has declined in relative terms.
These things depend on your circumstances. At Armstrong I was able to deal with the management, etc, on a reasonable personal basis and everyone know how business was going. Different world to the Defence company or a big Uni. Nothing like being van driver or delivery man for a big company that treats you as a 'subcontractor' or some other trick.
Nor, I suspect like some chains that look like a big UK company, but are actually 'francises' from a non-UK company that hoovers money away overseas as 'costs' in the UK to dodge tax and responsibilities here. Fill in with the name of your unfavourite maker of gadgets, opticians, chemists, etc, often charging premium prices.