  Recently, Paczos et al. (Phys. Rev. D 109 (2024)) proposed a covariant
quantum field theory for free and interacting tachyon fields. We show that the
proposed Feynman propagator leads to unitarity violation in the 1-loop mass
renormalization of an ordinary particle it interacts with, proper asymptotic
(in/out) tachyon states do not exist, and the proposed S-matrix describing
interactions of tachyons and subluminal matter is ill-defined. Since tachyons
behave as bosons, interacting tachyons may also self-interact, e.g., any
interaction with ordinary matter generates such terms. As a result, the
physical vacuum, instead of being at the origin of the potential, may
correspond to the proper minimum of the tachyon potential, or such state does
not exist at all. Our analysis indicates that quantum tachyon field does not
describe a physical on-shell particle with negative mass squared.
