Jepsen is an important sanity check on marketing. Easy to say in collateral you are "strict" ACID -- Jepsen proves (or denies) those claims. Some fun reports over there http://jepsen.io/analyses
It can't prove strict acid because it only observes behaviour it can observe. There are code paths it won't see in practice, so the best it can do is refute.
It also can't refute acid as a whole. If you look at the reports, they usually check the very basic behaviour - simple sets and gets. While that can tell you a lot about the system, you can't say it will generalise to complicated joins, multi-row operations, backend plugins, etc.
Consistency in the ACID sense meaning something different from CAP Consistency is another good example.