There are a few tools to make it easier though: if you're dealing with a site that uses a lot of semi-random subdomains (like googlevideo or cloudflare), it's possible to just whitelist the whole domain and get all the subdomains automatically (and then block individual subdomains if you want).
It might be defeating the purpose, but you can also change the "scope" of the changes you make by clicking the blue button in the upper left corner of the panel. You can choose the scope from current domain, current subdomain, or global "*". If you select global, you can, for example, unblock the various youtube/googlevideo related embeds/iframes/xhrs there, and then youtube embeds will work on every site.
The one thing I wish the addon would do is sync my black/whitelists through Firefox Sync so I didn't have to either redo everything on a new computer, or go to the trouble of exporting/importing them.
It's technically possible, although UX is terrible. Go to uMatrix settings, make sure to "enable cloud storage" option is set in the "settings" tab, then manually upload and download rules in the "my rules" tab.
No automatic sync (won't be implemented[1]) and, no merges at the moment (button's broken[2]).
By design. Losing all rules because of sync snafu out of control of uMatrix is the absolute worst case. Importing/merging manually will prevent such disaster.
As I get it, it would require significant changes to the code, though.
I personally just stick to base domains usually. When a site is broken, allowing one or two 3rd-party base domains is often just enough to fix it.
If you want to go subdomain-granularity, this is your choice -- it's not forced on you. Some even go as far as subdomain/type (the cells in the middle).
I myself though i had to enable each sub-set individually until i took a closer look at the behavior when clicking certain ui elements (that as i recall were unlabeled).
Noscript on the other hand only present a list of domains, though one can enable sub-domain granularity in the options (never felt the need).
[0]https://addons.mozilla.org/en-GB/firefox/addon/umatrix/ [1]http://adamantine.me/index.php/2015/11/18/umatrix-desperatel...