Clearly there is divergence. Examples that I run into on a daily basis:

My credit union login does an infinite redirect on login. Works fine on Chrome (and all other major browsers)

Perplexity on the mobile version web search entirely broken. Loops with some error and becomes unresponsive. Works fine on safari.

Many other random stuff breaks at least a few times per day usually from login redirects and authentication. Extensions like 1Password have autofill only working some of the time. The list goes on

Most of their failures are usually on their custom features instead of basic WebKit / Safari functionality, in my experience. As a basic browser it’s great, but I consistently have issues with 1Password logins, especially if passkeys are involved. That isn’t necessarily their fault but the end result of me being unable to use their browser is the same. And if I’m going to use a basic browser I might as well use Safari and not deal with any third party software.

They use a custom fork of WebKit with many features added