Not as readable as YAML/TOML. Vastly less if unindented.
More annoying to write than YAML/TOML, no trailing comma. Yes I'd rank it lower than YAML even with its whitespace annoyances, good editor handle that, JSON is still annoying.
Not super compact (althought it's not too bad when compressed).
No proper numeric types, just one "sorta int, sorta float, depends on parser, say farewell to NaNs and infinities", you can't even losslessly transfer a normal IEEE flaot across it.
The schema thing came late so rarely used or supported.
So it really is pretty mediocre all things considered.
Definitely "good enough" but could also be improved.
It really doesn't though.