  In some former works of Azzam and Tolsa it was shown that $n$-rectifiability
can be characterized in terms of a square function involving the David-Semmes
$\beta_2$ coefficients. In the present paper we construct some counterexamples
which show that a similar characterization does not hold for the $\beta_p$
coefficients with $p\neq2$. This is in strong contrast with what happens in the
case of uniform $n$-rectifiability. In the second part of this paper we provide
an alternative argument for a recent result of Edelen, Naber and Valtorta about
the $n$-rectifiability of measures with bounded lower $n$-dimensional density.
Our alternative proof follows from a slight variant of the corona decomposition
in one of the aforementioned works of Azzam and Tolsa and a suitable
approximation argument.
