What I like the most about this collaboration software is the simple and logical UI, which is not overloaded with lots of unnecessary features. It has all we need for successful communication.
Cross-platform applications ready to use. Amazing open source community built around the product, friendly and helpful people.
The software is easy to use, similar to the commercial app Slack. It provides good thread tracking and the interface is clean and easy to understand.
The incoming hook limitation of maximum content length is the hell (Mattermost don't inform the customer about a message which was higher above the hard coded message size limit).
For its purpose, I have found nothing about the software that I can find to complain about.
A bad user interface, a lot of bugs, sometime you need to restart it in order to make sure that everything works as expected.