Accessible complaints procedures are extremely important in every branch of manufacture, extraction, agriculture, production, service and commerce. (These categories overlap, but no matter.) Regrettably, the waters will be muddied if there is a strong union presence where you work. Unions are extremely prone to corruption, for two reasons.
The first is that there's a fundamental conflict of interest between the union executive and the rank-and-file. Say that there is a standard 36-hour week where you work, and overtime is sometimes available. If nine individuals are prepared to work an extra four hours, that will mean one less job, and for the union, one less set of dues. So the union will campaign against overtime, under the slogan 'Your overtime is stolen from someone else's job!'
The second is that in times of high unemployment, unions become the active enemies of efficiency. So, there may be two semi-skilled men working in a particular capacity, and doing it well because they know how. The shop steward will agitate for a third man to be taken on and trained, claiming the work is too much for two. If he achieves this, he will then agitate for one of the original two (his friend) to be promoted to supervisor/trainer. That will require the employment of a fourth unskilled man to take his place.
Thus, when times are hard, unions will actively work to bring industrial firms down, and increase unemployment. They don't think of it this way, of course; and when the firm they have undermined sinks, they will blame the government for not having subsidised it sufficiently by raising taxation.
As regards complaints procedures, it depends what you're complaining about. If your complaint is about some union-imposed racket, complain direct to the General Manager, with copy to the company chairman, marked 'for information'.
If you think you're being victimised by your supervisor, and you're a union member, complain to whoever he reports to, with copy to your union rep 'for information'.
Note that by doing either of these you will become a marked man. If you're wrong, you will very soon be shown the door. If you're right, someone else will. On the three occasions that I've felt motivated to do this in my current employment, at least three and possibly five people have been shown the door. I'm still standing, but as I'm regarded as dangerous, I have no prospect of promotion. Well, you can't have everything.