The key point is that a program can generate inputs far faster than you can. So a testing system based on generation of inputs can create and execute far more cases than manual creation of individual unit tests.
The problem becomes determining if the test has failed. You should be creating properties to test, not inputs.
There used to be, back in the stone age of computing, this notion that random generation of test inputs was bad because it would waste valuable computing time. It was thought to be better to manually create carefully crafted individual tests. But the relative cost of people vs. the computer has changed by many orders of magnitude since then.
The problem becomes determining if the test has failed. You should be creating properties to test, not inputs.
There used to be, back in the stone age of computing, this notion that random generation of test inputs was bad because it would waste valuable computing time. It was thought to be better to manually create carefully crafted individual tests. But the relative cost of people vs. the computer has changed by many orders of magnitude since then.