Flock vs Mattermost
When comparing Flock vs Mattermost, the Slant community recommends Mattermost for most people. In the question“What is the best team chat software?” Mattermost is ranked 6th while Flock is ranked 17th. The most important reason people chose Mattermost is:
You don't need to rely on someone else's servers, you can host Mattermost on your own server.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Cross-platform
They have apps for Windows, Mac, Chrome, iOS, Android and web interface. And all the chat sessions are always on sync.

Pro Clean and clutter free interface
Pro Cheaper than Slack
It is half the price of Slack and full of nice features like polls, assign to do's, etc.
Pro Very powerful integration APIs
Unlike its rivals, it not only allows you to receive events, but also allows you to push content to it and receive various user action events.
Pro No IT team required for setup
Moving a team or organization to flock does not require any setup from the company's IT team. At max, what might be required would be to whitelist their domain in case the organization's IT team has gotten external domains blocked. You can simply download Flock and invite others in your organization by using their email ids in order to start collaborating. That's all there is in the name of setup.
Pro Free if you don't need the most advanced features
Most of its features are free, even video conferences. But also they have a Pro version with unlimited chat history, Active Directory integration among other things.
Pro Unlimited groups and teams
There is no limit on the number of groups or of group members. Similarly, within a team you can have unlimited members. There is no limit of number of teams too.

Pro Faster than Slack
Flock is offering an app that is faster than Slack.
Pro Highly configurable permissions
For every team, the permissions and restrictions can easily be set over the participating members and the conversations they are having.
Pro Better than slack
Pro Stickers
Posting messages is more fun with them in addition to emojis.
Pro Customisable UI, integrations surface just like native features
There are multiple options like slash commands, message buttons, chat buttons, attachment picker etc., where the integrations can surface inside the native app. This greatly eases the discoverability of a integration, making it really easy for the team members to find and use the integrations.
Pro Can be self-hosted
You don't need to rely on someone else's servers, you can host Mattermost on your own server.
Pro One-line Docker install
With Docker set up, you can install Mattermost with the following command:
docker run --name mattermost-dev -d --publish 8065:80 mattermost/platform
For other cases, installation instructions can be found here.
Pro Similar to Slack
Almost all features of Slack are available in Mattermost.
Pro Free - OpenSource
You can download and use it as free for self-hosted server.
Pro Mobile apps for android and iOS
Pro You can change theme
Pro Desktop app for MacOS, Linux and Windows
Pro Browser notifications
Pro Mark-Down support
Yes, it supports markdown.
Cons
Con No thread feature
Reply command only quotes snippets. There's no implementation to show thread(s) of an initial message.
Con No easy End-to-End Encryption setup
Con Features not available out of the box
Difficult to setup the many features it offers. The easy docker installation is for the application only, not it's feature sets. Requires a license for full-set of features.
Con Centralized
Con Poorly made iOS app
Built natively using React it suffers from unresponsiveness, input lag, and sometimes broken services. Coupled with a bad user experience.
Con Poorly made Android app
The Android application is just a badly wrapped web-view which does not perform well and has no form of offline caching whatsoever.
Con Self-Hosted
This doesn't work for every type of company. Mainly great for those concerned with privacy, security, compliance and control of your data/information.
