That's the thing though, uBlock rules can also inject scripts and addons can intercept and replace contents of network calls through the injected code.

I think mitm proxies should be used for whatever people find them suitable. I don't have a problem with this use case, especially as this is clearly something you will only get working with a moderate understanding of the underlying concepts.

Tools like Privoxy have existed for years and I don't know why you wouldn't trust a proxy over an addon. Just don't set it up for other people who can't make the risk/reward judgement (though the same goes for adblockers and other extensions).