There is no need to philosophise about the meaning of "it works".

Evidence based doesn't mean the treatment works for 100% or even for 70% of clients. You might want to look up what it actually means.

The approach for ensuring objective feedback that I've read about uses databases compiled from observations about many clients and their progress in therapy. And then you as the particular client are given e.g. questionnaires about your symptoms periodically, and the changes in your symptoms are regularly compared to these databases to see how well the treatment is working if at all. I would say that's plenty objective even considering how psychology is a young science and all that and is much better than having nothing at all. After all, without actual symptoms, how do you tell there is a problem anyway? So those are measurable and you can evaluate progress based on enough experience is the idea. This has been done before, but apparently too many (?!) therapists don't like the idea of objective feedback. Lol. Even though it would help improve their approach, too.

There's also been enough research to show that therapists are usually biased about the treatment working, because they don't think of checking for problems. So that's also very important to keep in mind. Don't listen to therapists, listen to yourself as a client when it comes to how well the treatment has been working or if it's been working at all. You can note down for yourself how your symptoms have been changing, and evaluate them, even without these databases. Using objective data as evidence is recommended anyway when working with feelings e.g. if you are depressed and think that doing x will not make you feel better, you can try doing that thing and you can track how you feel on a scale, in each relevant situation. And what it shows might surprise you.

I could go on and on but yeah, it is just important to keep objective in psychology as with anything else. Even when you are working with feelings. Or it will all just get completely ungrounded and there will be risk of being exposed to quackery.