The reason that Zappa’s drummer auditions were “brutal” was simply that his music was brutal to play. Zappa’s compositions were unusually angular and often written in very unusual and complicated asymmetric meter (time signatures) and road maps. Most virtuoso (in the usual sense) drummers capable of laying down a great groove in 4/4 or 3/4 would not stand a chance playing many of Zappa’s compositions and would fall apart very quickly. Same with players on any other instrument.
FZ’s “Keep It Greasy”: Time signature alternates between 19/16 and 21/16 with a slow section (guitar solo) in 11/4…..AND then make it groove, or improvise a solo. Good luck with that!