I dug into this myself just now. At it's core, uMatrix is "default deny" so it doesn't need all the blacklists. But there are things that uBlock Origin provides that uMatrix does not perhaps including cosmetic filtering, popup blocker, zapper/picker element, etc. The author Raymond Hill (gorhill) has switched back to uMatrix himself and dedicates more development efforts to it now.

https://discourse.mozilla.org/t/support-umatrix/5131/179

I use them together, uMatrix as a fine grained control tool where I can setup a saved profile and click-allow until functionality & appearance are acceptable to me, and for 'one-off' or 'fuck-it no time' situations I just toggle uMatrix off. It is very rare I have to turn off uBlock Origin as well (the nuclear option). This leaves me four-five quick clicks away from the median or average user experience.

uBlock Origin offers part of the uMatrix functionality as well in the "advanced user" mode.

Yes.

Other than the issue of cosmetic filters (which I don't personally use), the main difference is that for uBlock you can subscribe to really good lists, collected and curated by a lot of people - so it is "defauly allow, except for these...", whereas uMatrix is "default deny, except for what you explicitly allowed".

I run both, occasionally have to allow something in uMatrix, never have to turn uBlock off.

You can configure uBlock to use default deny as well by changing the mode [1].

It's not as fine-grained as uMatrix but works similarly.

[1] https://github.com/gorhill/uBlock/wiki/Blocking-mode

Upvoted for the info, but I'll restate why I use both:

uBlock is my blacklist. uMatix is my whitelist. For various reasons, I occasionally need to skip the whitelist, but I never had a reason to turn off the blacklist in a few years of using uBlock.

They can do similar things, uBlock Origin is setup to block ads and other elements on the page (depending on the lists you subscribe to). But uMatrix is more like Noscript, it's blocking the loading of resources from the web. So you can block javascript from some domains, while allowing images or video, etc. I use both so i can block a lot of third party scripts even without them being ads (so most tracking scripts and such).