Firefox focus has an emphasis on privacy. That means no history and no bookmarks. No history means no cache. No cache means I have to refetch all resources every time I open it--if this is, say, a list of articles from a limited selection of websites, this is incredibly inefficient and will eat my limited data. No bookmarks or history means I can't use it as a primary browser to open common sites without typing them in--I can save shortcuts to pages in the launcher, but this causes usability issues (can't see entire title or URL; no horizontal view; my folders on Nova launcher are hierarchically organized drawer groups, which doesn't support shortcuts; can't search shortcuts only, results include deep phone search).

Firefox focus is a very good browser for opening links on WiFi or unlimited data when using another app. It's not very good at all at being a primary browser.