In the last few years, I have seen a number of talks and papers which tested for an effect of sonority sequencing on various phenomena, either synchronic or diachronic. Without exception, I think all these studies found a null effect, but in each case I was struck by how conciliatory and uncritical the author(s) were of the idea that sonority sequencing exists in the first place, given that they failed to find any effect of sonority sequencing in a domain where they expected to find one. I submit that the sonority sequencing principle is something of a zombie idea, a bad idea that just keeps coming back.
The idea of sonority itself is over a century old, but it has proved extremely difficult to ground in any physical reality or to provide a precise, generalizable definition of what exactly it is. This is probably because it doesn’t exist. At best, the construct we are trying to measure may be some kind of perceptual salience, which is weakly correlated with sound change (and thus with synchronic phonological processes which arise from sound change), but which is highly contextual and just one of many contingencies governing sound change.
The idea that sonority is a scale (properly, a ratio measurement in the sense of Stevens) is itself decades old as well, and gives rise to the idea that grammars constrain the differences in sonority between adjacent phones in specific ways, as in the principle of sonority sequencing. If we focus our attention on languages that permit tautosyllabic consonant clusters of any sort, I have yet to see a single case where syllable phonotactics are cleanly described by imposing thresholds on this principle. In nearly every case I have seen—Turkish is a famous example—there are many systematic gaps which cannot be explained with reference to sonority or to any other known cause beyond historical contingency (e.g., in Turkish, the absence of coda *[rn], *[lm] despite their favorable sonority profile). In such cases, I see no reason to give any credit to the sonority sequencing principle.
Sometimes, theoretical progress involves not just the introduction of good new ideas, but also the rejection of old, bad ideas. I think sonority is one of those old, bad ideas, and I think phonologists should view it with a much more critical lens than they currently do.