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Rear echelon are the most versatile engineering unit in the game.
Their mid-long range damage is similar to combat engineers yet far superior in close range.
Huge number of potential weapon upgrades(including doctrines), they can be kitted for anti tank, and anti infantry.
Doctrinal upgrades (rifle nades, 30cal lmg, flamethrower) give them all the more flexibility.
They can equip these weapons without giving up their minesweeper.
They can build cover and MG emplacements.
They have an(admittedly weak) suppression ability, though this will be stronger if you have them upgraded with weapons.
5 man vet improves squad repair rate, durability and damage output simultaneously without needing to purchase an upgrade or select a doctrine for it.
They MIGHT be slightly overpriced at 25 reinforce, but I couldn't see it being any less than 23.