IS with 1 bren/BAR-K will simply perform excellent in all range.
The Fuel requirement to do this makes this impossible in any reasonable game between two players of equal skill. Even if it were possible, it doesn't actually provide enough benefit to justify doing because you're directly contradicting Infantry Sections main strength of superior long range damage by removing one of their Bren Guns, additionally, because you're attempting to also be aggressive with this squad you're invalidating the strength of the 1 Bren Gun on approach resulting in a gimped close range squad that doesn't deal enough damage to justify closing in with and a weaker ceiling in the lack of a second Bren at long range.
Ro.E compared to RE might start a bit slow but once they vet up they become way too durable thus double BAR-K or even 3 weapons on them including flamer will make them a very cost efficient unit in late with lots of utility.
Unless I'm wrong, you can only get 2 weapons on the Engineers, both the Recovery Sappers and normal Sappers. Sappers do become durable at Vet 3 but at the point where they are Vet 3 and you've placed double Vickers K's on them they are no longer a supporting Engineer unit. You are using them as a Mainline Infantry Unit and that is okay. When compared to Infantry Sections they're just fine and you're trading a late game infantry performance boost for an early game performance deficient when compared to Sections, not only that, but you're denying the ability for handheld At and Minesweepers for the ability to do this which is a fair tradeoff.
In a similar manner Raid section with double bren (or brain/BAR-K) will also perform great at all range similar or even superior to LMG Riflemen.
Putting Brens on Raid Sections make no sense as you can simply build regular Sections and place Brens on them for infinitely more impact and better performance. The only justification here I suppose would be the ability to skip Bolster which would leave you with a weakened Tommy + Sapper for not much benefit. While again placing Bren+VickersK on a Raid Section runs into the same problem as with regular sections that you're taking a unit designed with a purpose in mind and neutering that purpose for general utility which is shown to not work because utility does not trump specialized strength.
Your fears are unfounded in any practical sense.