You need to compile Vivaldi with the Chromium codecs.
From here: https://wiki.archlinux.org/index.php/Vivaldi
WebMs work fine for me on Linux using Vivaldi as long as I install it with the ffmpeg codecs.
The note taking feature is fantastic. It's basically this process:
highlight text to take note of
right click and click "take note" or wtv
This creates a note that contains the text, a link to the webpage, and a screenshot of the page all at once.
The Side Panel is the other primary reason I use it. It also has built-in tab layouts. Like, if I group some tabs together, I can have the browser split the tabs across the window in columns. Useful if you own a 1440p or greater display and especially useful if you own an Ultrawide.