  We revisit the phenomenology of neutrinoless double beta ($0\nu\beta\beta$)
decay mediated by non-interfering exchange of light and heavy Majorana
neutrinos, in the context of current and prospective ton-scale experimental
searches, as well as of recent calculations of nuclear matrix elements (NME) in
different nuclear models. We derive joint upper bounds on the light and heavy
contributions to $0\nu\beta\beta$ decay, for different sets of NME, through
separate and combined data coming from the following experiments (and
isotopes): KamLAND-Zen and EXO (Xe), GERDA, and MAJORANA (Ge) and CUORE (Te).
We further consider three proposed projects that could provide, within current
bounds, possible $0\nu\beta\beta$ decay signals at $>\!3\sigma$ level with an
exposure of 10 ton years: nEXO (Xe), LEGEND (Ge) and CUPID (Mo). Separate and
combined (Xe, Ge, Mo) signals are studied for different representative cases
and NME sets, and the conditions leading to (non)degenerate light and heavy
neutrino mechanisms are discussed. In particular, the role of heavy-to-light
NME ratios in different isotopes is highlighted through appropriate graphical
representations. By using different sets of "true" and "test" NME as a proxy
for nuclear uncertainties, it is shown that the relative contributions of light
and heavy neutrino exchange to $0\nu\beta\beta$ signals may be significantly
biased in some cases. Implications for theoretical models connecting light and
heavy Majorana neutrino masses are also briefly illustrated. These results
provide further motivations to improve NME calculations, so as to better
exploit the physics potential of future multi-isotope $0\nu\beta\beta$ searches
at the ton scale.
