When Raymond Hill stopped working on uMatrix (uM) to focus on uBlock Origin (uBO), he disappointed quite few of us. I have always thought that uMatrix was easier to use despite that the reverse was supposed to be the case.
xiMatrix was born as a result of uMatrix being put into hibernation, the project being a simplified fork of uMatrix which has gained some well deserved attention. More recently a friend of mine ('Wrongthink') forked xiMatrix and created paraMatrix which promises to improve upon the feature-set of the former.
Several people, all smarter then me, have posited that uBlock is not capable of the granular filtering offered by uMatrix and therefore the latter is still necessary for us privacy advocates to which i have argued that anything you can do with uMatrix you can do with uBlock, however dong so is not as simple as it is with the point-n-click UI of uMatrix and instead requires the creation of custom filters (see the "My Filters" tab of the UI) which can block resources on a per-file basis, or per-file per-host.
At one time i saw both uBlock and uMatrix as necessary, the former being configured to handle static blocking and the latter for dynamic blocking (until very recently i always thought the 2 played well together). As vastly more powerful privacy protections were built into Firefox however, and given my obligatory usage of the 'arkenfox' user.js, i began to question the necessity of uMatrix and eventually dropped it, even from my "advanced" Firefox privacy guide. Part of my reasoning was "the fewer extensions, the better" logic and part of it was due to the greatly enhanced anti-fingerprinting tech added to Firefox, as well as state partitioning which isolates browser storage (cookies) per host.
Yes, uMatrix offers easier and more granular protection over uBlock from its pop-up interface with its ability to control CSS, cookies, XHR and media with just a click of the mouse rather than having to write custom filters, but i saw no point in blocking CSS, which will transform the majority of websites into a pile of unintelligible rubble, or 1st party media which is no longer an annoyance given the option to block auto-play in Firefox.
Regarding the cookie filtering ability of uMatrix, this storage can be controlled with Firefox natively on a per-host and per-session basis if one desires, though i personally don't see an overwhelming need to bother with this any longer given that this storage is siloed (for the most part) assuming one has enabled Enhanced Tracking Protection (ETP) and set it to "Strict".
Both xiMatrix and paraMatrix have extended the capability of uMatrix to include font filtering and i think that alone may make them quite attractive. As for me, i do this with a custom filter in uBlock (see my uBlock Origin Suggested Settings guide if interested).
Regarding graphical content, i think it's nice that paraMatrix separates images from the media category which xiMatrix doesn't do. I'm not sure that's of value to me personally, but i'm sure others will appreciate it.
So given that i always configured uMatrix to allow CSS globally and allow all 1st party images, media and frames, and given the remaining overlap in the functionality offered by uBlock when it's configured to operate in its advanced mode, and given all the other functionality offered by uBlock, especially the filter lists, it didn't seem to make make much sense to continue using uMatrix.
I would also posit that the modern web, being the shit-hole that it is, is only going to continue to get worse and maintaining a reasonable sense of privacy, much less anonymity, which is already essentially impossible, is going to continue to get harder and harder. If one wishes to continue accessing the clear-web, one is going to be forced to make concessions. That said, anyone concerned with privacy (and everyone ought to be) can and should mitigate as many threats as they are comfortably able to given their threat model. As for me, i can see myself largely abandoning the clear-web at some point in the future and moving to some distributed tech that isn't affected as much by corporate centralization and censorship.
Having said all that, i still miss uMatrix and i personally think that Raymond made a mistake by not incorporating the complete uMatrix grid into uBlock Origin. That disappointment aside, it's nice to see others picking up where Raymond left off and creating some redundancy in the web filtering department in the event he "gets hit by a bus" as 'Wrongthink' stated in his paraMatrix introductory post. The potential pitfall with these filtering extensions is that they're largely dependent upon the Manifest v2 web extension specification which is being phased out by the jackasses at Google, but which Mozilla has promised to continue to support... for now. Given that Mozilla derives a huge amount of income from Google however, and given all of the other retarded shit they've pulled off, i don't put any stock whatsoever into their promises.
In my opinion what is really needed is a fork of uBlock Origin with the uMatrix grid, but i suspect that's a massive undertaking. Failing that, i think it's great to see projects like xiMatrix and paraMatrix, and though i don't see myself using them in the near future, i am certainly not ignoring these projects. I'll be keeping an eye on the development of paraMatrix particularly.