Onion USB isn't something you build a team around like Cloud USB, but it's still very good.
Right now, if you're bringing Onion, he's probably kind of dead weight in your team when he does BSB for buffing, then something like breakdowns or entrusting.
With his USB (and legend dive that comes out at the same time), he will instead give even better buffs, but now he will be one of your good damage dealers also. The mage USB is party quickcast, mag/mnd/res boost, and haste, but it also gives Onion an EX mode that gives him an additional (stacking) +30% magic and +15% damage with black magic, and his LD gives him 35% dualcast to all black magic. You definitely want him to be dropping Meltdowns and Chain spells instead of something like Fullbreak after he's dived.
The physical USB is good too, but his physical side only gets dualcast Celerity, which basically means dualcast Dervish because other celerity skills are bad for DPS. The USB is a direct upgrade to shout though, providing 50% party ATK and haste, along with 50% party crit chance. It also gives Onion an EX mode that gives him 30% extra ATK and quickcast that keeps getting faster with every skill used for the duration.
His LMR on the same banner gives him initial instantcast2 which lets him get his buffs online even faster than before.