  We perform a comprehensive analysis of scalar contributions in $b \to c \tau
\nu$ transitions including the latest measurements of $R(D^{(*)})$, the $q^2$
differential distributions in $B \to D^{(*)} \tau \nu$, the $\tau$ polarization
asymmetry for $B \to D^{*} \tau \nu$, and the bound derived from the total
width of the $B_c$ meson. We find that scalar contributions with the
simultaneous presence of both left- and right-handed couplings to quarks can
explain the available data, specifically $R(D^{(*)})$ together with the
measured differential distributions. However, the constraints from the total
$B_c$ width present a slight tension with the current data on $B \to D^{*}\tau
\nu$ in this scenario, preferring smaller values for $R(D^*)$. We discuss
possibilities to disentangle scalar new physics from other new-physics
scenarios like the presence of only a left-handed vector current, via
additional observables in $B \to D^{(*)}\tau \nu$ decays or additional decay
modes like the baryonic $\Lambda_b \to \Lambda_c \tau \nu$ and the inclusive $B
\to X_c \tau \nu$ decays. We also analyze scalar contributions in $b \to u \tau
\nu$ transitions, including the latest measurements of $B \to \tau \nu$,
providing predictions for $\Lambda_b \to p \tau \nu$ and $B \to \pi \tau \nu$
decays. The potential complementarity between the $b \to u$ and $b \to c$
sectors is finally investigated once assumptions about the flavour structure of
the underlying theory are made.
