1. Scale. A pure function taking just one int input would require 4 billion tests. This means few hundred gigabytes of test source code and several months of execution time. Square it if a function takes two ints. For String it practically goes to infinity.
   
   
2. Assume we have these tests, executed on a quantum computer or something. How do you know the expected result for each particular input? You either enter it by hand (good luck) or generate expected output. By generate I mean write a program that produces expected value for every input. But aren't we testing such program already in the first place? Are we suppose to write better, error-free version of code under test just to test it? Also known as ugly mirror antipattern.
   
   

1. Absolute value of any number should never be negative
2. Encoding and decoding any string should yield the same String back for every symmetric encoding
3. Optimized version of some old algorithm should produce the same result as the old one for any input
4. Total money in a bank should remain the same after arbitrary number of intra-bank transactions in any order

Spock + custom data generators

def 'absolute value of #value should not be negative'() {
    expect:
    value.abs() >= 0

    where:
    value << randomInts(100)
}

private static def List<Integer> randomInts(int count) {
    final Random random = new Random()
    (1..count).collect { random.nextInt() }
}

import spock.genesis.Gen

def 'absolute value of #value should not be negative'() {
    expect:
    value.abs() >= 0

    where:
    value << Gen.int.take(100)
}

import static net.java.quickcheck.generator.CombinedGeneratorsIterables.someLists
import static net.java.quickcheck.generator.PrimitiveGenerators.integers

def 'sum of non-negative numbers from #list should not be negative'() {
    expect:
    list.findAll{it >= 0}.sum() >= 0

    where:
    list << someLists(integers(), 100)
}