I run an XMPP server for my friends. We use Conversations [0,1] on Android and BBOS, and Zom [2] on iOS. We use OMEMO [3] for encrypting most of our conversations, and while it isn't perfect, it usually stays out of the way.
Generally, the experience with the mobile clients has been quite good. Conversations and Zom are stable, attractive, and featureful. The biggest issues are some interoperability problems with desktop clients (displaying messages that should be hidden) and some things which I believe are server-side configuration issues.
Zom hides a some useful configuration features (in the name of being dead-simple to use), so I'm trying to convince one of my iPhone-owning friends to try ChatSecure [4].
I run my own Mattermost server and the mobile version is very lookalike the Slack one (I only use chat so I don't know if there's other functionalities on Slack that are not in Mattermost)
Generally, the experience with the mobile clients has been quite good. Conversations and Zom are stable, attractive, and featureful. The biggest issues are some interoperability problems with desktop clients (displaying messages that should be hidden) and some things which I believe are server-side configuration issues.
Zom hides a some useful configuration features (in the name of being dead-simple to use), so I'm trying to convince one of my iPhone-owning friends to try ChatSecure [4].
[0] https://conversations.im/
[1] https://f-droid.org/packages/eu.siacs.conversations/
[2] https://zom.im/
[3] https://conversations.im/omemo/
[4] https://chatsecure.org/