1, One of three abilities that turn MEC's into irreplaceable units. Augment Heavies (-20 AIM and no CRIT), use boosters to get the high ground (+ your aim and defense) and then go on overwatch (+ 15 defense) even while on offense and your MEC's have huge durability.
2. On normal difficulty almost everything dies in two hits from MEC regardless and the few that don't die in two hits the extra 4 damage means spit while the other is very useful most of the time, and against the hardest enemies in the game it is a life saver (missile spams for example will greatly nerf its damage due to the first missile hitting before the others when proc'ing damage)

3, This is one of three things that either makes people using MEC's say "they are OP" vs "they are trash!". Don't use your MEC as a bullet wall. Use boosters and make sure you ALWAYS have high ground bonuses. Period.
4. Repairing 6 points of damage TOTAL over a mission? Utterly. Worthless. Having more grenades and mines? OP.

5. With "Body Shield" you won't/shouldn't be taking critical hits (if you are fighting more than one group of people at once you aren't playing right). This means no one in game does more than 33% health in single hit. No-brainer choice.
*. Always augment HEAVY soldiers as their added ability "Body Shield" makes MEC's the second most OP class in game. Period.