The false comfort usually comes from line coverage. I had a model at 97.2% coverage, 92 tests. Ran equivalence partitioning on it (partition inputs into classes, test one from each) and found 6 real gaps: a search scope testing 1 of 5 fields, a scope checking return type but not filtering logic, a missing state machine branch. SimpleCov said the file was covered. The logical input space was not. The technique is old (ISTQB calls it specification-based testing) but the manual overhead made it impractical until recently. Agents made it possible to apply across 60+ models, which is the one thing they have changed for testing so far.