By critiquing his reluctance to accept patches because he'll be stuck maintaining that code? By forking Vim and removing some compatibility and legacy support features when tarruda's multithreading patch was not accepted?

No, the core contributors have not been outright in their criticisms, but IMO the fork itself shows a lack of respect for Bran's work and a lack of willingness to work with him to make Vim better. And now tarruda has stepped back from NeoVim, perhaps recognizing himself the cost that Bram bears.

By simply contributing to the tests - one of the outright critiques of Vim itself - they could have brought a lot more confidence to accepting such patches to Vim, but they chose to take their ball and play elsewhere instead.

What frustrates me the most is that instead of two branches of Vim that are moderately better (with the future of one of the branches being left behind), we could have had one which was much better.