  The LIGO/Virgo gravitational--wave observatories have detected 50 BH-BH
coalescences. This sample is large enough to have allowed several recent
studies to draw conclusions about the branching ratios between isolated
binaries versus dense stellar clusters as the origin of double BHs. It has also
led to the exciting suggestion that the population is highly likely to contain
primordial black holes. Here we demonstrate that such conclusions cannot yet be
robust, because of the large current uncertainties in several key aspects of
binary stellar evolution. These include the development and survival of a
common envelope, the mass and angular momentum loss during binary interactions,
mixing in stellar interiors, pair-instability mass loss and supernova
outbursts. Using standard tools such as the population synthesis codes
StarTrack and COMPAS and the detailed stellar evolution code MESA, we examine
as a case study the possible future evolution of Melnick 34, the most massive
known binary star system. We show that, despite its well-known orbital
architecture, various assumptions regarding stellar and binary physics predict
a wide variety of outcomes: from a close BH-BH binary (which would lead to a
potentially detectable coalescence), through a wide BH-BH binary (which might
be seen in microlensing observations), or a Thorne-Zytkow object, to a complete
disruption of both objects by pair-instability supernovae. Thus since the
future of massive binaries is inherently uncertain, sound predictions about the
properties of BH-BH systems are highly challenging at this time. Consequently,
drawing conclusions about the formation channels for the LIGO/Virgo BH-BH
merger population is premature.
