In talking about testing in 2021 Tim Bray says explicitly about Integration or end to end testing …
The problem is that moving from monoliths to microservices, which makes these tests more important, also makes them harder to build. Which is another good reason to stick with a nice simple monolith if you can. No, I’m not kidding.
Which in turn means you have to be sure to budget time, including design and maintenance time, for your integration testing. (Unit testing is just part of the basic coding budget.)