The differences are marginal. Yeah you're much higher level, but you'll still take damage (admittedly less, but you can still be downed by a group of lv 3 enemies if you just rush in and let yourself get shot)
Damage output against lower levels doesn't seem to increase. Low level fallen vandals still take 2 scout rifle head shots even tho I'm lvl 24.
It seems like gaining lvls only helps you do full dmg against higher lvl enemies, not extra dmg to lower ones.
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Plus loot is based on your lvl. So going back to early game can still get you upgrades. A lot of people farm earth/moon just trying to get legendary engrams to drop because they get more frequent and less challenging kills.