It's not too bad on modern Ubuntu-based variants in my experience. There's a font manager GUI that lets you drag and drop add stuff, one click install font, etc. like on Windows. You can also just copy ttf or other files into your ~/.fonts for your user and they work fine too (I like this since you can just sync and backup your fonts across all your machines).
The real mess, sadly, is configuring all your apps and such to use different fonts. There are an unbelievable amount of obscure configs, tweak tools, etc. if you want to change system fonts.
Once you start accumulating all the fonts you needed over projects, your Inkscape and Gimp become terribly sluggish on booting. Other programs maybe too, I don't know, I don't use that many graphical tools or office programs.
And the font-selector dropdowns become unusable.
So I started manually moving stuff from /usr/share/fonts/ and ./.local/share/fonts/ into per-project archive folders. A lot of work, that a font-manager could easily do much better. Worse: manually moving stuff from /usr/share/fonts/ can and will bring them back "unexpected" on updates or reinstallation - though there are probably hardly ever updates for font packages.
I would love a tool like fnt to manage this for me, but it seems fnt is more for installing and less for "dis- and enabling" them per theme/project etc.
> Once you start accumulating all the fonts you needed over projects, your Inkscape and Gimp become terribly sluggish on booting.
Isn't this an old Windows bug, where GIMP would recreate the font cache on every start? I don't think I've ever seen any noticeable delay on Linux. (I have hundreds to thousands of fonts installed.)
It boots fast, but many menu's and buttons will stall for seconds or longer while fonts are being loaded. There may be an issue for this in some bugtracker, I have not checked this yet.
The real mess, sadly, is configuring all your apps and such to use different fonts. There are an unbelievable amount of obscure configs, tweak tools, etc. if you want to change system fonts.