People have different definitions for "working". I assume that the people who claim that Matrix doesn't work are the ones comparing to alternative commercial solutions and all the polish that usually accompanies those.
replyA decade ago Moxie Marlinspike argued[1] that to develop Signal more quickly, he and his team could not be distracted by federation. At the time I wasn't sure what to think, but it does seem that the effort level of using the integrated approach of client/server/protocol may just logistically be easier.