Google people regularly tell me that qualified candidates have a 50/50 chance to make it through the interview process successfully. I'm sure they're working on that, but I would be thinking about it in those terms.
If you don't prepare the probability is much less than 50/50. I would rather have prepared and failed then just go it there and bomb it. You are going to have waste a day going to interview might as well be prepared.
I didn't say not to prepare. :) Just defining the terms. Ie, a "good" chance is 50/50. I think knowing that might influence how you prepare, how you go into it mentally -- and most importantly how you process a rejection if that's the outcome you get.
That's kind of the point -- even if you prepare well, your chances are only 50/50, so don't be too disappointed if you don't make it.
These interviews are no different than a lottery where you hope and prey they choose an algorithm question you studied and learned well
A recruiter at Google literally told me which algorithms to know study and learn well, and only one of them came up in interviews (DFS).