You have to learn how to use them and their abilities.
Cruise Chaser is very flimsy but it's strong if used properly. It moves fast so you always have to be on the move and attack accurately. You have 3 attacks, a close range, a cone and a target AoE but that takes time to cast so you have to anticipate the location of the target granted it's easy to attack slow moving Oppressors or to get them when they are using their missile attack.
Like the name suggest, it's a Chaser, you literally run and move towards targets slashing them and keeping them on their toes. it's a very AoE based attack so it's easy to disrupt a large group of players if they are bunched up together.
Oppressor is mainly a Tower crusher however if you take the North Path the bridges and the overall higher terrain is a great place to use it's secondary action to push opponents off potentially killing them, it's something to keep in mind when using them.
Brute Justice, you have to know how to use their ability, Flamethrower and Mega Beam are very strong to use to take out players but that should only be your secondary objective to prevent dmg on Brute Justice. Your main objective as Brute is to crush Towers with Rocket Punch, it does the same amount of dmg as Oppressor in the time you can reuse Missiles making it a very strong mech for turning the tide of combat.
The biggest issues players have currently is they focus to much on taking players down. You only want to take out players with your mech is if they have soaring to weaken their strength. When you are in a Mech you shouldn't use it to attack players unless the players are trying to attack your Oppressor but if there is another Cruise Chaser that should be your first target.
Overall it' just day 1 so people don't know to properly use the mechs so to suggest they are disposable from one day is a bit of a leap in assumption since there are a majority of people who haven't read what the mechs actually do.