I just want to add that if you don't get detailed feedback, please don't take it personally, and neither should you push harder to try to get it. I'm not a recruiter or an HR person; I'm a business owner who sometimes needs to hire someone to join our firm. Here's what that process looks like for me: first, there's all kinds of calculating and number crunching to establish whether or not we really can afford to do this, including trying to guess what the market is paying at the moment, writing the job description, spreading the word, etc. Then, there's a huge wave of resumes, many from people who can't even spell the technologies they claim to know. I have too much work anyway - that's why I'm hiring - but I have to find the time to extract a short list from the pile. Then I (or if I'm lucky, someone I delegate to) must get in touch with the short list and schedule the interviews, deal with having someone on my short list already hired elsewhere, etc. Then the actual interviews, which can be kind of a blur sometimes but I take a lot of notes. Then selecting a first choice, making an offer, which might get accepted but might not. Possibly making an offer to the second choice. Finally writing to the rest saying we hired someone else. Then busy training the new person and trying to catch up on whatever backlog accumulated while I had to hire.
And then candidate #3, or #7, emails and wants feedback on what was not great and what could be better and how to improve? Look, I'm a really helpful person, you can see that, I spend time here, I blog, I speak at conferences, you know? But there's a time and a place. And right after finishing an exhausting round of hiring is almost never the time. I sometimes tell people "you outperformed several other candidates, but were not the highest ranked." The chances of getting more details than that are slim. It's different in a big company where they might want to keep you on file for another position. It's certainly very very different for a recruiter who wants to place you and wants to improve your interview performance. But for me (and plenty of other interviewers), it's a favour you're asking, at a time when we may not be able to grant it.
That said, I wouldn't hold it against you to ask. I just might not answer usefully.