The top 3 answers to the MathOverflow question Which mathematicians have influenced you the most? are Alexander Grothendieck, Mikhail Gromov, and Bill Thurston. Each of these have expressed serious concerns about the community.

• Grothendieck was actually effectively excommunicated by the mathematical community and then was pathologized as having gone crazy. See pages 37-40 of David Ruelle's book A Mathematician's Brain.

• Gromov expresses strong sympathy for Grigory Perelman having left the mathematical community starting on page 110 of Perfect Rigor. (You can search for "Gromov" in the pdf to see all of his remarks on the subject.)

• Thurston made very apt criticisms of the mathematical community in his essay On Proof and Progress In Mathematics. See especially the beginning of Section 3: "How is mathematical understanding communicated?" Terry Tao endorses Thurston's essay in his obituary of Thurston. But the community has essentially ignored Thurston's remarks: one almost never hears people talk about the points that Thurston raises.