  Electron shuttling is one of the currently pursued avenues towards the
scalability of semiconductor quantum dot-based spin qubits. We theoretically
analyze the dephasing of a spin qubit adiabatically transferred between two
tunnel-coupled quantum dots. We focus on the regime where the Zeeman splitting
is lower than the tunnel coupling, at which interdot tunneling with spin flip
is absent, and analyze the sources of errors in spin-coherent electron transfer
for Si- and GaAs-based quantum dots. Apart from the obvious effect of
fluctuations in spin splitting in each dot (e.g., due to nuclear Overhauser
fields) leading to finite $ T_{2}^{*} $ of the stationary spin qubit, we
consider effects activated by detuning sweeps aimed at adiabatic qubit transfer
between the dots: failure of charge transfer caused by charge noise and
phonons, spin relaxation due to enhancement of spin-orbit mixing of levels, and
spin dephasing caused by low- and high-frequency noise coupling to the
electron's charge in the presence of differences in Zeeman splittings between
the two dots. Our results indicate that achieving coherent transfer of electron
spin in a $10\,\mu$m long dot array necessitates a large and uniform tunnel
coupling, with a typical value of $ 2t_c \gtrsim 60 \, \mu$eV.
