has already been answered. Do not send the rear echelon alone fired at the front to the fuel without the fighters capturing at the adjacent point or going to the same capture point as the rear echelon. They are really weak and consider that they will always lose at x1 if the opponent has not passed through some penalty ground on defense like a river before getting close to you. Rear echelon weren't meant to take on anything alone in the early game. I play this way and never lose an early checkpoint against a lone pioneer. They usually send him in for vision and with the MG 42 close behind to do the dirty work.
I think you're misunderstanding the topic, the question isn't if RE are bad or why they are bad, but why they cost so much for being that bad.
RE were nerfed some time ago (april 2016) because one player used to spam them efficiently and win games in auto-match then balance team being what it is they applied the usual hammer nerf on them not trying at all to re-balance them.
Rear Echelons
We have scaled up Rear echelons up slightly to reduce spam.
Rear echelons cost up from 160 to 200
Rear echelon rifle damage up from 8 to 10
I like the slightly.
Then they received smoke as a nerf for USF since rifle lost it at the same time.
Then they've been nerfed again with the no dual M1919 for them and rifle (which was then true initial problem)
Then they lost 2 dps for a bit more accuracy. The true real reason was they were too good vs light vehicle with their 10 dps and no accuracy penalty. Kubel would melt and they could deal some damage to a 222 behind cover. another nerf in reality.
Then they received with the new doctrine the grenade launcher.
Somewhere in between they also received the flamer upgrade because removed from riflemen (another nerf), which has been a disaster for the commander relegating it into oblivion.