There are some criticism lodged against their operations, in that some employees are not worker-owners, but simply workers. I'd call this "slack", and I'd suggest "slack" is largely an artifact of a lack of other cooperatives. There is of course the fact that buy-in does cost money, which I would argue would necessarily set a floor for "tenure" in many cases.
Having said that, they have evinced a certain special robustness in the face of crises - in the 80's (iirc) for example, rather than laying people off, they sent them into their education programs.
https://www.youtube.com/watch?v=zaJ1hfVPUe8 https://vimeo.com/161252994