My experience was that the interviews were easy and fun once my interviewers got over the embarrassed "this is really stupid, but I'm supposed to ask you this really trivial question...".
The recruiter I dealt with, on the other hand, was collossally incompetent, and ranks among the top reasons why I declined Google's job offer.
After you join Google you never have to deal with the recruiter again and the interviewers become fun people to work with.
OTOH an incompetent recruiter can result in a bad job offer and I can understand why that could be a big negative.
It is hard to fix problems if nobody reports them.
In all of the companies I have worked in as an employee or with as a consultant, the HR department always has by far the weakest staff. I have also observed that the number and degree of problematic staff in non-HR departments is largely correlated with just how weak the HR department is. This makes complete sense, of course, since it is the HR department that is supposed to address such problems and it is the HR department which usually plays the largest role in disciplinary decisions and firing decisions. The department holding onto firing decisions will always have the weakest staff, and for the simple reason that they aren't exactly going to fire themselves, now are they? :) The rest of the company's staff problems simply fall out from that. Not that I am suggesting that Google has notable staffing problems, rather simply that no matter where you go, even Google, the HR department (or whichever department in reality owns hire/fire, usually HR) will necessarily be the weakest in the organization.
I'm sure other people have had different, less positive, experiences. I'm also quite sure that there's a selection bias among those who feel they need to publicly talk about their interview experience. But, yes, it's a big company and there's bound to be mistakes and mishaps and bad interviewers and recruiters.
The two biggest issues were (a) not understanding that a DPhil qualifies as "Master's degree or better", and not bothering to ask what it was; and (b) changing which job I was applying for three times without asking me.
When I turned up in Mt. View for my interview, I was greeted with "so, you're interested in a job doing X", to which the only response I could provide was "... I am?" My first interviewer did a heroic job of rearranging my interview schedule to make it more appropriate, but obviously there's a limit to how much you can fix on the interview day itself.
I don't think it's incompetence per se. They just interview people all the time and try to apply the usual Google techniques of scaling (which involve acceptable failure rates). Recruiters are sometimes staff and sometimes not.
A good engineer would have caught the problem right away, but apart from your phone screen, no good engineer looked at your resume until maybe five minutes before you were in the building. This isn't carelessness as much as mental self-protection. People are super busy there and they still do a lot of interviewing.
The system works best if you are a generic Stanford grad with no fixed ideas about what you wanted to be doing. If you are even a little bit unusual the gears can suddenly seize up.