Even if you set aside universality as a desirable property, according to the official site at 128kbps any Opus efficiency advantage disappears. https://opus-codec.org/comparison/
That graph does not describe codec efficiency. It's explaining how the various codecs preserve the frequencies of the raw signal at various bitrates:
Some codecs only work at low bitrates and preserve only narrow bands of frequencies. Some codecs work only at mid bitrates and preserve wider bands. Some codes only work at high bitrates and preserve only the widest bands; you can't get then to drop more frequencies for better savings even if you wanted to. Opus works on all bitrates and gradually and dynamically removes frequency bands as the bitrate drops. Vorbis preserves more or less the same frequencies as Opus at the same bitrate, but loses frequencies a bit faster as the bitrate drops. MP3 drops even faster. AAC works very similarly to Opus, but can't output low bitrate streams.
To compare codec efficiency you would need to do subjective comparisons to see how often each codec achieves transparency (when people can no longer tell if the sound has been compressed or not) at a given bitrate with various types of sounds. This has also been measured, and it's agreed that Opus is basically transparent at 128 kbps. MP3 needs twice as many bits to get the same quality, so Opus is twice as efficient.
My friend, the vertical axis is literally labeled "Quality", and the horizontal axis "Bitrate". The caption is "The figure below illustrates the quality of various codecs as a function of the bitrate." Quality at a range of bitrates is how codec efficiency is measured.
I'd never heard the claim that "Opus is basically transparent at 128 kbps", but I did find https://wiki.hydrogenaud.io/index.php?title=Opus, which agrees with you: "Very close to transparency". But it also notes, "Most modern codecs competitive (AAC-LC, Vorbis, MP3)", which lines up with the chart.
Early Opus vs. MP3 tests were done with LAME, which is awful. This may be why you're under the impression that MP3 needs twice as many bits to get the same quality.
And the labels on that axis make it perfectly clear what they mean by "quality". It's how much of the spectrum they preserve at that bitrate. If "quality" referred to subjective quality there's no reason why the chart should stop at 128 kbps. It stops there because the fullband codecs don't brickwall the signal past that point. Instead they use psychoacoustics to compress it.
>Early Opus vs. MP3 tests were done with LAME, which is awful.
That's funny, because other commenters say LAME is currently the benchmark for MP3 encoders.
Which is why I use Opus128 on my iPhone, where storage is a limited, fixed commodity and my listening environment is rarely optimized. Everywhere else is FLAC or mp3-256. My ears aren’t golden enough anymore to justify 320 bit mp3.
