In terms of the class + mirror,
I truely belive the classmechanic is what is holding the career down. The turret have waaay too many goodies tied to it. And dies waaaay too easily. Some Magus/Engineers will find sneaky spots for each fighting area to hide their turrets around corners and still get their stacks. But that just paints the picture of how immobile the class is, restricting it be in certain areas or overly focus on keeping the pet in range while trying to read a battle, when to prekite, when to control, when to burst. No other mechanic really demands this sort of attention from you. Sure IB/BG might come close but the difference in an engineer with stacks and one without is Massive!
As for the engineer vs the magus,
Most engineers who get invited into 2-2-2 and escape the pug-hell are adapting into a more utility role. Magnet being one second slower than Magus rift, half of the ability arsenal being corp/psyical, giving up your class mechanic stacks for applying a ghetto meleerange pbaoe knockdown and fully forfeit being a "real damagedealer" in an attempt on giving Order an aoe knockdown to setup moraledrops or even prevent them for the cost of 1-2 tactics and yet the turret still explodes before 2GCD in a deathball fight
Magus being able to snare and prekite with Firestorm, while Engineer is stuck in place during a P.Shells channel is an other reason engi is falling behind in terms of damage+utility role.
Overpopulation of Engineers,
That IS a thing on Order. And I hate to say this, but the quality of the engineers I've seen on my time on RoR is shocking. And allow me to elaborate on that comment. Sure alot of the engineers who have trialed and tried to apply to the orvr guilds on order on an engineer spot have had good, heck even amazing, career knowledge. But straight up terrible awareness or ability to read a battle. And that is the main issue why most warbands for city just dont want a single engineer. The highest rank engineers on the server know their turret mechanic and bonuses in their sleep, but then have no clue how to control a battlefield with their tools and be a "control mage" type of role. The engineer/magus toolbox is massive with control, debuff and utility tools. Yet most engineers only understand how to stand out of Detaunt range with Snipe, with no concept of pre-kiting, debuffing the target(s) who are getting focused while interrupting or stopping the backline and controling an income flank attack. Its like the Bitterstone Uniform came with Blinkers (for a horse) on, with how deep the tunnelvision is for the most engineers ive seen on RoR.
On top of my head I cant think of just THREE decent control engineers who would be worth a DPS spot in City, partly down to their classmechanic being too easy to shutdown, but also down to lack of the above.
Giving the career + mirror a "loner" tactic where they just get "X"% damage and able to cast their version of Self-Destruct around them with no turret, but unable to get the amazing bonuses for each of the turret options like armordebuff, range, disrupt, dotduration, targetlimmit extention etc etc. Would maybe allow them to perform on a DPS role and not just a utility role. So its a tradeoff option to run with or without the pet turret.