This notion that there is a wide distribution in engineer competency has come under a lot of criticism for many reasons (not the least of which being that its often used as a cover for discrimination on the basis of gender): https://lwn.net/Articles/641779/ If you believe there to be such a wide distribution in talent and that salaries at your company are commensurately paid, I suggest you ask your employer if they will disclose every employee's salary and the justification for them.

Even if this we take this for granted, its completely untrue that all unions commoditize a trade by flattening pay. There are plenty of unions that have chosen not to put standardized salaries in contracts or have advocated tiered salary agreements or merit pay. Whether or not these are good things, they are things that unions have done and should dispel with the notion that unions are inflexible towards these concerns by workers.

As for your concern that unions "force employers to do things", well I suggest you consider all of the ways your employer can coerce you to do things and whether you actually have a mutualistic relationship. Many workers don't feel that they do.