PvE core engi is really bad compared to elite specs. Whatever core could do, elite does just better.
If you want to play relaxed and tanky, you go scrapper. Even if you only spam bombkit #1, scrapper is the spec you prefer because of the insane barrier gain.
If you want to max out your DPS, you go holo. Even condition builds are straigt better with holo.
There is no reason to play core engi except nostalgia.
This is sad, i always loved engi but i never liked the high-tech light and laser aesthetics of holosmith. It is kind of strange to holo mode with all that high-tech skills and then switch to kits with its rough look. Well this is really not important for gameplay, but it never felt "correct" to me.
