The engineer and pioneer are required to construct building to tech, but in return, they can be upgraded with flamer. Other faction cannot unless specific commander is selected.
I would argue if upgrading to flamer counterweights dual LMGs or nades or faster repair rates or actual combat potential and combat scaling on top of repair scaling.
CEs and Pios got nothing on RETs, spios and REs who can be actual combat troops with actual combat scaling.