The EBU provides a set of sample PCM audio files to audit loudness measuring equipment. Based on testing a few random files from the set, ffmpeg's EBU R128 analyzer passes. But you may want to do a thorough survey.
Result for 1kHz Sine -20 LUFS-16bit.wav
Integrated loudness: I: -20.0 LUFS Threshold: -30.0 LUFS Loudness range: LRA: 0.0 LU Threshold: -40.0 LUFS LRA low: -20.0 LUFS LRA high: -20.0 LUFS
Result for 1kHz Sine -26 LUFS-16bit.wav
Integrated loudness: I: -26.0 LUFS Threshold: -36.0 LUFS Loudness range: LRA: 0.0 LU Threshold: -46.0 LUFS LRA low: -26.0 LUFS LRA high: -26.0 LUFS