I empathize with everything you're saying here, but I do humbly disagree. I've experienced exactly what you're talking about, and it's a nightmare. Frankly, I think the Rails codebase itself even suffers from an over use of concerns that can make figuring out where functionality comes from very frustrating at times.

That said, concerns are mixins, and mixins are, in the simplest of terms, more or less multiple inheritance. That's powerful stuff, and powerful stuff in the wrong hands (which includes every hand that'll touch it, not just the author!) will always get you in trouble. That could be the motto for Rails itself - powerful stuff, but in the wrong hands it'll get you into trouble!

My first exposure to concerns in Rails was a awful codebase for a well known company with massive classes that someone went on a weekend bender and shrunk down to less massive classes by introducing dozens of concerns. It made the classes _appear_ smaller, but they weren't actually. In reality, it was just hiding away much of the complexity and horrible decision making that made the code so hard to reason about and maintain and took things from bad to worse as a result.

That said, when you have a well architected code base to begin with (as in before you start introducing concerns) that has svelte classes with appropriate responsibilities, with shared logic broken out into "services" or whatnot, concerns can be a really nice way to share isolated functionality that can lead to cleaner classes without obfuscating what's going on. Quite the opposite - used well, they can tuck away (but not altogether hide) ancillary functionality allowing the meat of a method to be highlighted instead of drowned out by pre- and post-requisite behavior that might take up more LoC than the unique business logic of a given controller action thus obfuscating the functionality specific to that method.

On my current and previous project, concerns worked out great for the teams (which included junior devs) but were used sparingly and typically introduced by experienced devs. Also, taking a quick glance, it seems like Controller concerns are favored far more, and often involve calls to class-level ActionController methods such as rescue_from, overriding protect_from_forgery defaults, settings layouts, before_action, helper_method, etc. - things that you can't just tuck into a service class or method that gets called from the controller and would otherwise require someone remembering the correct combinations of code to sprinkle into a controller or creating a controller to inherit from (which isn't practically different from concerns but can paint you into a corner). They're also never used for the business logic of an action, only supporting functionality (again, pre- and post- requisite functionality or introducing class-level helper methods that can be reused).

All that being said, for a team that isn't confident in how to use concerns effectively, I'd also steer them away and guide them to consider alternative approaches. But at face value, I don't consider concerns an anti-pattern and have found that, when used appropriately, they can be a great tool in maintaining a clean codebase.