It is a strong benefit, but it is also the only benefit at level 3 of the subclass, and it works only on your allies.
I do agree it skews combat priorty towards making as many attacks as possible, instead of a lower number of more powerful hits (the same tended to be the build priority of Sharpshooter builds in O5E with Crossbow Expert dual Hand Crossbow builds), and people like myself will probably turn it into a numbers game where you calculate at which target AC you should or should not use this. But then again: Even though dual wielding was buffed, many classes have other uses for bonus actions, and there is a very limited number of combat maneuvers that increase the number of attacks you can make in a round. With Archery Fighting Style now only available to Fighters and Fletcher Rangers, and Battlemaster's Precision Attack not in the game, the accuracy hit of -5 is not as easily trivialized, and at early levels the payout is not as great either (2d6 < 10). Sure, that's where you have your Marshal powers and Mark Foe reduces the penalty to 2.5, but on a class that is designed to improve the power of allied combatants, I feel this is not as terrible as "everyone gets Sharpshooter for free" would be in O5E.
I tend to agree that the passive nature of this being somewhat strong. It would not have surprised me (comparing power levels of other subclasses), if this benefit would only be an option for attacks made via Commanding Presence, though that would limit its use tremendously, or that it would cost some sort of action or resource to enable it for the round.
Problem remains: Marshals depend heavily on party composition. In certain parties they are ok, in others borderline overpowered. This makes balancing the Marshal across the board very difficult. Your party getting one or two attacks per round with Daring Assault is slightly underwhelming for a subclass. Them using it 10 times in a round is very strong.
Finally: Yes, Scorching Ray is likely the most disgusting you could go with this, and it gets worse. At lvl 17 with Critical Weakness a full caster could spend a 9th level slot to make 10 Scorching Ray attacks, all with advantage, -5 to hit and with 3d6+6 extra damage per hit. Assuming a lvl 17 Sorcerer as the caster, Intense Spell metamagic will turn this into max damage rolls, meaning 10 x max[5d6+6] = 360 damage, assuming all hits. You might think this is ridiculous, but even without a Marshal close by, Sorcerers of this power level could cast Meteor Swarm with Intense Spell for unmitigated accuracy that deals 168 damage in four 40 ft radius spheres. Still, being able to almost oneshot an Ancient Brass Dragon is pretty sick.
EDIT: One more thing about power on a third level sorcerer: Targeting 3 enemies with Shatter without a Marshal nearby deals roughly the same amount of damage, but without the -5 to hit and dealing half on a miss. Strong, but not overly so I would say.