The main reason I dislike the assert libraries is that you lose the ability to format things nicely; being able to see what's going on quickly is pretty nice when tests fail. Sometimes you want %s, sometimes %q, sometimes maybe a diff or formatted as something else. Testify "solves" this by just dumping a lot of stuff to your terminal, which is a crude brute-force solution. Plus with table-driven test you're saving what, 2 or 3 lines of code?

For example for a simple string comparison it's 12 lines:

   --- FAIL: TestA (0.00s)
       --- FAIL: TestA/some_message (0.00s)
           main_test.go:11:
                   Error Trace:    /tmp/go/main_test.go:11
                   Error:          Not equal:
                                   expected: "as\"d"
                                   actual  : "a\"sf"
   
                                   Diff:
                                   --- Expected
                                   +++ Actual
                                   @@ -1 +1 @@
                                   -as"d
                                   +a"sf
                   Test:           TestA/some_message
 

vs. 2 lines with a "normal" test:

   --- FAIL: TestA (0.00s)
       --- FAIL: TestA/some_message (0.00s)
           main_test.go:11:
               have: "as\"d"
               want: "a\"sf"

With your NotNil() example it's 4 lines, which seems about 3 lines more than needed.

This kind of stuff really adds up if you have maybe 3 or 4 test failures.