Personally, I like using multiple columns, although not the way Mastadon's "advanced" UI does it. I just use a Userstyle to expand what would normally be a single column into multiple columns.

Usually, I modify whatever normally occupies left/right sides with a narrow column that expands when I hover over it. This means that most of the screen width is available for the columns of the main stream, but the functionality present in the side columns is still available.

It depends on the site. I have multi-column userstyles for Diaspora, Mastodon (works with both "Advanced" and basic UI, in different ways), Tumblr, MeWe, and Facebook.

Of these, Tumblr and Facebook have CSS changes often enough that I need to fix them up relatively frequently, although it has actually been some time since the last time I needed to update them.

The one which has the most refinements is Diaspora, since I use Diaspora most heavily. For the others, I've tried to keep my modifications to a minimum, so it'll be less likely to break things and less effort to maintain.

The screenshots don't show the "hover" activity in action, but anyway here are my userstyles:

userstyles.world/user/isaackuo

The screenshots are a bit out of date also.