The fundamental difference between Coherence and Unite is the backend engine used to power your apps.
Coherence is chromium-based, and can use a variety of browsers, including Chrome, Brave, Edge, Opera and Chrome Canary. Because Coherence uses cross-platform browsers by third-party vendors, it is more powerful internally, but limited in additional OS-level functions.
Unite is built on WebKit 2, the same engine that powers Safari, and uses the custom, BZG-built Unite browser to power your apps. While not supporting all of the extensions and functions of Chrome, it supports a number of OS-level functions that make your apps more 'app-like' on macOS.