Your company probably talked to our company a lot through the years. It's been a while, so I don't remember my specific complaints, but I do remember that push notifications would sometimes not be delivered, so I would miss messages. It would also go the other way. There are a lot of small nice refinements in slack & discord that mattermost just doesn't have.

Once recent new feature slack added that I like was scheduled messages, but that is a pretty new thing.