..trusted web proxy
@Lee D
If I want to install an addon that can alter all sorts of things then that is done at my risk - the browser is welcome to give me hideously dire warnings about possible security risks, but should not prevent me installing what I want, at my own risk.
If I install backend software to do "blocking" I still have to put some trust in at that stage instead, e.g. if I run Privoxy I could look at all the source code (just like I could for browser addons such as UBO etc) but probably won't, I will just "trust" it (albeit with safety of knowing plenty of technically skilled people will have inspected it, but we all know code inspections do not always spot every issue))
So, risk no matter what stage I have some "adblocking" functionality, and for non technical users I would guess a browser addon is far more likely to be used than e.g. proxying software (& setting it all up), also if we think about "easy" options such as Pi-Hole, for those using ISP low functionality / locked down routers, then adding a basic "1st level" protection such as linking up a Pi-Hole may not be easy for them.
Browser addons are a (IMO) a good way for your non tech savvy relatives to have a safer browsing experience (& I know from bitter experience if you do something more complex for them, if anything ever goes wrong on their system (even though totally unrelated, e.g. a classic "break lots of things" dubious MS update) they always blame you ... so I long since stopped being IT tech support for relatives! ).
Given that websites are flinging all sorts of nastiness at me, then I would sooner use a browser addon I trust (to some degree of trust) rather than just let a website fling all sorts at me & be unable to do nothing about it (we assume I have no backend protection for sake of this argument, i.e. bog standard user).
Disclosure: I use UBO, NoScript & other defensive addons, plus some backend IP/DNS filtering too, but backend stuff not much help when some ads / nasty content served from DNS I may want to allow (tempting as it is (as lots of malware makes use of Cloudflare & similar CDNs to appear innocuous - a Cloudflare URI serving content looks more trustworthy than e.g. evilmalware.com URI) So I cannot ban all Cloudflare IPs as some sites I want to visit use it, hence NoScript is useful for fine grained control as I can see what is served from where and choose to allow (or not) script from a cloudflare URI).
NoScript is, for me, the most important browser addon for security reasons (with heavy use of JS in ad serving it also indirectly blocks a majority of ads)
For me ad blocking via UBO is secondary but mainly for
1. Performance - no / few ads makes a big difference to navigating around the web.
2. Stops pages jiggling around - with async loading, many web pages "move" as yet another bit of ad related content is loaded, this jiggling around increases likelihood of clicking on the wrong content (as page rearranges itself just as you click, so you click on something you did not want to). By having an "immobile" web page without async ad loads, it reduces chance of an accidental nasty click.
3. A long and indefensible history of ads being used to serve malware, so basic sensible browsing to try and protect yourself (if ads had remained, few in number per page & as a small bit of text (or image) with a link then there would be no need for all this, most of us only began to block ads when it became a JS frenzy with large amounts of large space consuming ads per page)