What percentage of PhD theses are rejected nowadays?
I'm only personally aware of one student who failed his PhD defense (this is at an R1 US university). After his advisor refused to approve his thesis, he went over his head and got the department chair to schedule the defense anyway. Results were predictable.

On the other hand, "major revisions" are very common, especially, I hear, in the humanities (in engineering, it's far more common to receive token feedback -- if the committee reads the thesis at all! -- than demands for substantial changes).

Outright failing a student during a defense is an extreme embarassment, for the department, for the PhD committee, for the advisor, and of course for the student, so there is every incentive to ensure that a thesis that goes to defense will pass. Moreover, since most theses these days are compilations of previously-published work, it is very easy to tell well in advance if the student is expected to pass.

So if an advisor has doubts about the quality of a student's thesis, he will either ask the student to spend more time improving it, or "suggest" the student start looking for jobs in industry.