There are reasonable security measures in place to ensure that uBlock Origin itself from PureOS is tamper-proof or at least very hard to tamper with.

But the block lists are pulled in from all over the internet and if an attacker manages to compromise any one of those lists on any one of the third-party servers that host them or while the lists are traveling across the internet, your privacy and possibly your security are as good as gone. I'm not even sure whether those lists are properly audited in the first place.

I'm not a security expert and the description might not be correctly formulated but I think a security expert can still get the idea.