Now, depending on the size (and architecture) of your system, the full test suite of into tests could take like, 10 hours.

You'll eventually have to run these 10 hours no matter what. If you only run the unit tests you won't be able to tell if your code actually works, and if your full e2e suite passes then you won't learn anything of value from running the unit tests - if one of them fails you can be fairly certain that it is that unit test itself that is broken, not the product.

During development, you don't need to run the full test suite. It's usually quite trivial to identify a single integration test that checks the parts you are touching, and then you only have to run it to see the effects of your changes.

And even then, you’re STILL probably mocking something (direct seeding bombs into the db). Full setup of all life cycles of all your entities using their associated entities (bombs, wires, wire clippers, bomb diffusers, their rank, their bosses, their diffusion history, etc…)

Integration tests rarely (if ever) need mocks. If they need Test Doubles at all, they'd typically be _Fake_s rather than _Mock_s. But even that is not always needed - an integration test can often just use non-production versions of the real dependencies.

So - if your real software typically runs against a database that sits on a powerful server and has tables with billions of rows, the e2e tests can use the same database engine but run it locally and only put a few dozens of rows in these tables. This setup is quick and cheap to set up and tear down while still being faithful to the production setup - maybe not 100%, but orders or magnitude more faithful than a unit test's mock that just compares the SQL commands given to it with the ones preprogrammed into it.

Another thread to pull on is what are you mocking in your tests to begin with, and if they’re not resulting in valuable test results then that goes back to one of my original points being that you might have suboptimal patterns in your production code.

Like, what logic governs bomb diffusion and where does it live? Are you mocking the right thing? If you have to mock the wrong thing… WHY? The ritual of writing those tests give you the first whiff of a potential smell.

My argument is that this is more about the "problem domain" than about the "solution domain". Some field of software work very neatly with unit tests - general libraries are a good example. Other fields are a very poor fit, and trying to shoehorn them into using unit tests - even worse, shoehorning them into using mostly unit tests - will inevitable result in a nightmare of mock maintenance.

I tell my guys not to test implementation: test behaviour. Test contract adherence. If that’s hard, no problem, message me let’s review it. Maybe a behavior they’re trying to test legitimately should be an int test. More often than not, their design is demanding to mock something that should be decomposed further, and then composed with dependency injections that are where the units that should ACTUALLY be mocked are the things that get mocked and injected, so the unit tests are actually testing the BEHAVIORS we want to pin rather than implementations. Composition and single responsibility principal are what enable unit tests to pull their weight.

The problem is that unit tests "promote" aspects of implementation into behavior. Classic example - the exact syntax of the SQL query a function uses to obtain the data it needs is by all rights an implementation detail, but a unit test is forbidden from using a real database so it passes a mock that expects a very specific query - now it became a behavior.