I think it totally depends on the expectations of the system, how much experience you have, and the risk you can afford.
One of the systems I've been working on for about 14 years is a CRM. It's probably about a million and a half lines of code with a few hundred movable UI components. At one point we had around 10,000 unit+integration tests but have removed many of them. The issue is it tolerates a fair amount of mistakes in edge cases as they typically don't impact many employees at a time because someone with knowledge of the business tested the feature before it went into production. My goal is to try to provide the business with a high ROI for development time and over the years I've seen what works and what doesn't. These days I typically use very few unit tests. In fact given the application I try to limit situations where I feel they are necessary at all causing fewer errors, faster turn around etc.
Most of my development is a UI, maybe some business logic, a model, and a DB. If I have tests I make a class with dependency injection just to test the business logic, and many situations there is no business logic to test. If there are very few critical paths or few people are using it I may also not add unit tests.
Financial portions of an application typically receive many more tests.
If you are a new developer unit tests may be useful to help you understand the potential issues with the patterns you use.
Unit tests don't replace integration tests. Or making tests to reproduce bug reports (seems you missed it the first time).
I think most new developers make things complicated enough to require tests because they are bored or because they think it is clever or they just don't think long term. It may unintentionally function as a training exercise. In my experience systems with a few repeated patterns over and over seem to stand the test of time much better. Much of the "complicated" code just comes from tried and true libraries you shouldn't be editing. The best business code is something someone else who isn't even a great programmer can sit down at and understand quickly so they can add additional features that are valuable to the end user.
You may however be in a very different situation. If you are writing a library to release, your company screams at you for every little error, your software will be installed in hardware and sold, it is customer facing, it is life or death, etc then I would change the way I write and test it accordingly.
As a note one way I reduced complexity (and unit tests) was to not be afraid to move complicated things to the administrative user space where possible. This also allows the business to build groups of people who may not be "programmers" but who can set up complicated business logic on a test server, test it, and move it to production without a developer even being involved. Your software will be more resilient to change. Along those lines I recommend moving anything that looks like a report to it's own department or at least its own thought process/repo.
/rant (since they canceled our fireworks due to rain) lol.
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