Thats is maybe best to answer with a small table:
| Mattermost | WebMeetings | Meet | |
|---|---|---|---|
| Text chat | Yes | Yes (not stored on the server | No (not yet) |
| Video chat | No | Yes | Yes |
| Integrated with WebApp | Yes, dedicated plugin | Yes, dedicated plugin | Yes, intranet plugin for now |
| Mobile phone support | yes (without plugin support) | No | Yes, PWA |
| Remarks | UCS app likely soon no longer supported by Kopano | Product already discontinued by Kopano | Successor of WebMeetings |