It's mostly just Sylvanas who suffers from this plotline though, since it makes her come across like the most bafflingly incompetent and moronic broad this side of WotLK Arthas.
The rest of the characters do reasonably well. Anduin being corrupted like Arthas, though against his will, is a wonderful plotline whose only weakness is being linked to Sylvanas.
Bolvar is given the most lore since at least WotLK, and it is solid enough.
The base concept of SL is not that great, which might be what constitutes a greater problem in SL that it is likely impossible to untangle from. But compared to the absolute travesty that was the plot pileup in BfA I am willing to give SL the benefit of the doubt and say that SL cannot possibly be as awful.
BfA will always hold the crown for me on absolute worst plot, simply because it had my ideal setup yet managed to squander it so bad. A faction war with a naval focus and possible intrigue on why the war was even started is a wonderful and seemingly simple way to describe BfA,and yet it squandered it at every turn, peaking with the ending that simultaneously didn't actually give us a conclusion to the initial plothook, and also ruined a long anticipated villain by rushing into it.
SL still only really suffers from the Jailer being boring and Sylvanas being stupid. The Elune thing was explained perfectly well, just not really exciting enough to warrant the mystery. And SL as a whole is slightly creaking under the weight of a concept it cannot really support, but other than that most of the plot is perfectly serviceable, certainly not significantly worse than any other plot.
In summary BfA will always be worse, barring a similar plot suicide from SL.