I'd really appreciate a letter with feedback after a lengthy interviewing process, I think that's really valuable.
However, I don't think I'd value it after more than 3 interviews. Candidates are applying after work hours and there's a time / energy limit to how many processes they can take at a time. After a given point, you're wasting not hours but possibly months of their life.
That's a fair concern. We really try and avoid advancing anyone when we suspect might be rejected in the middle or end of our process. It has only happened once since I've been here. Again though, I have to lean back on that everyone should be aware that job hunting isn't an ideal process. Occasionally someone can really ace one part of an interview and really not be a fit in a different part. It's unfortunate when it happens, and that's when I lean on my principles of offering constructive feedback.
I didn't really realise it until I was on the other end, but if you don't get any feedback you can totally ask for it and there's a decent chance you'll get some.
But I agree, it's usually pretty obvious if you just weren't good enough, or you had a bad day, or they just didn't like you for some random reason.
However, I don't think I'd value it after more than 3 interviews. Candidates are applying after work hours and there's a time / energy limit to how many processes they can take at a time. After a given point, you're wasting not hours but possibly months of their life.