Engineer is a medium armor class known for its tools, kits, elixirs, turrets...lots of fancy mechanical thingies that it utilizes for various DPS, support and CC-effects. Engineers also have unique "toolbelt"-mechanic which works as another skillbar on top of normal weapon skills. Skills on the toolbelt depend on utilities, with Bomb Kit as utility adding Big Ol' Bomb to toolbelt, for example. Since engineer has various kits to replace weapon skills, it can only use one weapon set at a time. But with kits as utilities, engineer can have whole 35 skills available at once and thus skilled engineers are known to swap between kits fluently to dish out some massive damage.
Engineer can be considered somewhat tankier than other medium armor classes thanks to highest healthpool. But even then, it may feel difficult to survive as especially condiengi needs to pay attention to swapping kits efficiently and this easily comes with a cost to situation awareness.
Condiengineer, like condiranger, is one of the vanilla builds that are still popular after all these years. Meanwhile, power builds have been way less used though the power-related traits did get boosted lately.
Engineer also has some healing capability, thanks to elixir gun and med kit, but healer-engineers are almost non-existent since it requires squadmates to pick up the med kits for efficient heals.
With Heart of Thorns, engineer got a support/power damage elite spec "Scrapper" which added hammer and gyros to engineer's toolkit. Its Function Gyro allows engineer to revive allies and finish foes from distance. Trait-wise Scrapper is focused on increasing power DPS, gaining more tankiness or granting superspeed to allies. Especially superspeed combined with stealth gyro is very useful for getting the squad from point A to point B fast and stealthily.
Scrapper has been mainly decent for PVP (though rare even there) due to its block and stunfield, but due to hammer's damage and only one general damage trait, it never became popular in PVE.
Path of Fire introduces a new Power DPS-focused elite spec "Holosmith" with a new heat-class mechanic, Exceed-elite skills and sword as a new engineer weapon. Holosmith replaces F5-toolbelt skill with Photon Forge, which allows the engineer to use powerful Photon Forge-skills while increasing heat level and if Photon Forge overheats too much, it explodes doing damage to holosmith and cooling down until the heat has fully dissipated. Heat also grants extra effects to sword skills.
Kitty recommends almost heating the Photon Forge to explosion point and disengaging just 1-2 attacks before explosion so it can cooldown a bit while improving sword attacks and using Photon Forge again when the heat has cooled to around 60 heat units.
As such, Kitty expects Holosmith to become somewhat popular among players, at first at least. Kitty hasn't properly got to compare Holosmith to other DPS-builds yet, but according to little testing she did the power DPS Holosmith should do rather good DPS, especially with Photon Forge engaged.