How do you tell that LastPass has been vetted better than the alternatives? "Experts looked at X and found no major issues" normally doesn't get very much attention, and potentially isn't published at all, or just reflected in recommendations.

(some of it is of course also depends on the threat-model: something with no (or with more caveats, an explicitly activated browser integration) is way less likely to be vulnerable against attacks launched from a website, reviewed or not)