I beat the game as a tank engineer on elite with some.
They are ridiculously durable. The one skill that creates a health bubble for you and all your allies is responsible for the bulk of it. Maxed out and at full charge, it was easily 4 times my own health pool. The construction options in engineer can make it fairly mindless if you want, though it won't clear super fast. basically it you keep all your robots up and spam spider mines, things will eventually die.
The one thing I didn't like about engineer is that I felt it suffered from bad game design. The shield is too good, to the point where almost every build has it. When mobs start hitting hard, you almost have to always have it up if you plan on taking any hits, and it's easily 60%-80% of your hp, and you can't see how hurt your shield is until your gone, at which point you have 20% of your life (your full hp) remaining, easily 1-2 shottable. The summons also have incredibly annoying cooldowns. Summoning a minion every 90 seconds that last only 60 seconds is fairly annoying. It's a long enough cooldown and type of ability that you want to set it and forget it in any given fight, but it's so short that you have to constantly refresh it, basically demanding that I keep checking the cd timer every 15 seconds. I also don't understand the need for a period where you don't have your summons, it seem widely unreliable. Every now and then you'll be in a fight where your summon is gone and not yet off cd, and everything dies extremely slowly until that timer goes away. It's a pointless condition. summons should be either entirely permanant set it and forget it spells or short durations with big effects and long cds.