Mainly thinking 48V as that is already a standard in the marine and automotive world. AFAIK 120V DC is not a commonly used standard ? (Maybe in aviation?)
replyMost modern aircraft have a 400V 360-800Hz three-phase grid, as this allows thinner (read:lighter) conductors. There are some high voltage DC rails on some plains in the range 270VDC or ±270VDC.
replyOn the usage side for instrumentation, avionics etc. it is usually 28 VDC.
50V is the limit for "safe extra low voltage" rules. Considered to be touch safe.
reply1. It is not considered safe voltage, so you have all the downsides of the current AC grid with the downsides of high DC voltages
reply2. Nearly no equipment uses 120VDC by itself, so you would need specialized DC-DC converters that can deal with the 120VDC input to go down to usual voltages like 48VD, 24VDC or 12VDC