When comparing Communi vs Polari, the Slant community recommends Polari for most people. In the question“What are the best IRC clients for UNIX-like systems?” Polari is ranked 12th while Communi is ranked 14th. The most important reason people chose Polari is:
The CSD-based UI is great to look at. No screen space goes entirely wasted and is presented neatly.
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Pros

Pro Elegant and minimalist interface
Communi user interface for desktop and mobile is minimal, which helps you focus on the task at hand instead of being distracted by extra UI elements.

Pro Good ZNC integration
The author is involved in ZNC development, so the latest ZNC goodies are guaranteed to be well supported (http://communi.github.io/znc/). With help of the documented ZNC modules, Communi retains its state well across sessions. No repetitive buffer playback on reconnect, notifications and banners for newly received messages only, queries stay open until closed etc.

Pro UI runs on multiple platforms
While Communi itself is a IRC framework, the project offers several UI implementations such as desktop, SailfishOS, Meego and Symbian.

Pro Quassel protocol implementation
Communi is integrated with Quassel.

Pro Event merging
Consequent events (joins, parts, quits etc.) are merged to reduce noise and to keep focus on the discussion.
Pro Great UI
The CSD-based UI is great to look at. No screen space goes entirely wasted and is presented neatly.

Pro Easy to use
Polari is very straight forward to use.

Pro GNOME-Integration
Polari integrates nice with GNOME, on a visual and technical level. It uses GNOME online accounts and runs empathy in the background.
Cons

Con No DCC
Communi does not support DCC (for a reason)

Con No scripting
Communi has no support for scripts
Con Some advanced features are lacking
Polari's users will find some advanced features lacking, such as Direct Client-to-Client (DCC).
Con May seem too simple
Unlike Hexchat or other alternatives, it does not expose much of its functionality though some of it might really be present.
Alternative Products
