I am a regular user of both chrome and ungoogled chromium browser. here are my insights: even though both are fork of chromium, UC consume much less ram as compared to chrome. a youtube page will take ~450mb in chrome but same page will take ~280mb (tested personally). You can install all chrome extensions in UC, there's also a seperate extension for updating these extensions. you can't "sync" data in UC (bookmarks, passwords, cards etc). there's no straight forward way to update UC.
