Okay so i tried recreating your Tests on the awakened Server on normal Class Sakura.
For the Engineer I used the lvl 75 Mcritd Light Armor and a lvl 120% +14 Gun with Temple Knight Accessories.
For the Mage i used the lvl 75 Mcritd Cloth Armor and a lvl 120% +14 Staff with Temple Knight Accessories.
1. Test: MDPS Engi vs 70 Fire Resist
So in this first Picture you can see my Mdps Engineer attacking a Templar with 70 Fire Resist. SuperIronMan is active(as well as some other buffs) letting me deal 17,5k Initial Damage with a Bonus of 6k Dmg due to the Oilbomb setup. So a total amount of 23,5k Damage.
2. Test: MDPS Engi vs 34 Fire Resist After Factoring in some Elemental Reduce (2x lvl 60 Mdps Ring and maybe a Heaven Fire Ring or Neondo T60 Trophy) the Templar should have around 35 Fire Resist instead of 70. we simulated this by removing the VL/DD Trophycombination, the Trophyenchants and the Whirlpool Cape:
in this second Picture we see my mdps Engi and the Templar with 34 Fire Resist. SuperIronMan is in effect - Enabling me to deal 33k Initial Dmg with a little Bonus of 11k by the oilbomb setup. That's 45k Damage - "Well that's a lot of Damage!"
However, lets compare this with an equally well equipped Mage;
3. Test - Comparison with a Mage
in this picture the Templar also has 34 Ice Resist. so the same as the fire resist when testing with the mdps Engi. Here the Mage does 39k Damage which... Is less then the Engineer? Yes. but only by a very small amount, and remember: The Engineer needs SuperIronMan AND Oilbomb Setup to deal equally as much damage as a Mage does with every single cast not being reliant on any buffs or setup at all.
Result:
Just like Jordan said; the Class could be usable in PvP but it's too reliant on the buffs and oilbomb setup making it inefficient or even unusable as your oilbomb can be cleansed and your SuperIronMan be removed or interrupted by enemy hits when buffing.
Oh and another Idea; Why does the immobilize Skill from Slime Bomb get removed when the Enemy is attacked, but Snowstorm does not? It's just a much weaker version (though instead of buffing Slime Bomb, i'd rather have Snowstorm nerfed with that Immobilize skill as the Mage already has enough options against Melee attackers...)