It really isn't. They can spend some of their billions of revenue to review changes when popular extensions are updated, just like Mozilla does. Every uBO update is vetted by Mozilla and is only then pushed out to users. But doing this is not in Google's interest at all.
Mozilla’s guidance on this (https://support.mozilla.org/en-US/kb/tips-assessing-safety-e...) is that only some extensions are manually reviewed and you shouldn’t trust this as a guarantee of safety if you don’t trust the developer who owns the extension.
