i'm almost through my Insanity Vanguard playthrough(using Energy Drain for its protection & make-up anti-shield capability). For an Engineer
- Carnage is probably slow to recharge, can detonate Incinerate (but so can other biotic like Reave, Warp)
- Fortification slows power use and boost melee. (melee for an engineer, you don't say)
- Marksman can be useful if you're a good aim, may benefit Soldier/Infiltrator weapon-based classes better.
- Inferno grenade is not bad.
- Defense Matrix (as for all cast-till-dispel damage protection skill) slows power use which is generally bad for powers-reliant classes like Engineer/Adept/Vanguard/Sentinel.
- Decoy is rather excessive considering the presence of combat drone & sentry turret that comes with offensive capabilities.
- Proximity Mine with enough power recharge bonuses can be use to setup a minefield (in theory: haven't tested this out) .
- Armor-piercing ammo allows you to shoot through cover and increases damage to health & armor.
- Warp Ammo is a less ideal as the +damage to lifted targets require active casting of Pull, Lift Grenade -- by your squadmates.
- Stasis doesn't affect all armored foes, for which all other foes barriers/shields, the engineer cam deal with. Using Incinerate on armored foes is preferable.
- Defense Drone hovers around you and is rather useless, because at max range of 10m (equivalent of un-upgraged Vanguard Shockwave) it is still rather lacking + damage is mediocre.
- Energy Drain is very similar to Overload, but E.D has active impact radius 3m vs. chain overload of up to 2 targets within 8m (@ 60% less damage)
- Barrier like Fortification & Defense Matrix is unfavorable due to slowing down power usage.
- Reave is a damage-over-time biotic that is outshone by Dark Channel.
- Dark Channel is a DLC bonus power, that at minimun rank last 30sec. If affected target is killed while D.C is active, the spell will jump to a new target (new duration unconfirmed) . It is a useful spell to prevent regeneration of shields/barriers for tougher enemies (e.g. Atlas).
- Slam is useless due to bad biotic detonation positioning and poor damage against all protected units. It also has no additional functionality unlike Pull, which exposes enemies at difficult angles or remove Guardian shields (both mentioned easily countered by Combat Drone)
Since an engineer will tend to balance out his/her team with a (half/pure) biotic & some pure weapon damage teammates, & extensive harrassment summonables are available, i would recommend armor piercing ammo for heightened personal effectiveness and/or squad effectiveness (if you choose the squad ammo bonus at 50% effectiveness).