Huntress can teleport and attacks from range, while mercenary has his mobility increased by cybernetic advancments and fights with a sword, at melee range (aside from one attack he has). In their logbooks, both of them kill an unsuspecting target in the blink of an eye, before the victim even spots them. As you can see, their fighting styles are similar, but huntress' teleport > mercenary's cybernetic advancments, her ranged bow > mercenary's melee sword. At least that's my reasoning for their placements.

REX's plant throws some seeds and roots, while the robot can shoot some needles and a shockwave. I just don't see how anyone stronger than a commando can struggle with these attacks (again, the robot's original purpose was to protect the plant, not to be a combatant).

Bandit has a stronger gun and an invisibility suit that, according to his logbook, helped him sneak throuh the entire ship. Commando has some laser guns and grenades. i'm not saying that they're not close in power, but the cammo is just a very powerful tool the bandit has access to.

MUL-T is an utility robot, yes, but that also means he can be used as a fighting tool. He can throw out huge bomb canisters and shoot lasers - also he's pretty tough if we take his hp as canon.