What happens to them on the field is due to your atrocious standard of play.
Royal Engineers have significantly higher dps AND higher defense than Pioneers. Maybe you're using your Royal Engineers in long range gun fights vs LMG Grens and raging when you lose.
In any case, the Royal Engineers aren't even a combat unit - they're primarily a repair and utility unit (minelaying, flamer, minesweeping, terraforming).
Have you considered not being so bad at this game? Anyone who thinks Pioneers are good at combat is probably on par with the Normal AI in skill level.
I don't use Royal Engineers in combat except as PIAT carriers and even at that they are terrible since the PIAT is the worst hand held AT in the game, and a Panzer 4 will consistently wipe 2-3 of their models in one shot.
I play mainly USF and UKF in 1v1 matches but the latter is almost unplayable now due to a balance team repeatedly giving in to Wehraboo moaning over the last few years.
Yes I've made my opinons known on the COH2 Steam forums too.
The .50 cal is a poor MG due to its very narrow arc requring either another MG to be positioned next to the first one to mitigate flanking (not great due to their high 7 pop count a piece) or you have to keep a rifle unit near to protect it. This is required far less often for an MG42 and even the MG34 because of their wide 120 degree fire arcs.
UKF was originally supposed to be the Allied version of Wehrmacht with costly, but high quality units and a few specific and exploitable weakness like no mobile artillery, and no anti-vehicle snares. Now its units while still being expensive, are either roughly equal, marginally better, or i n some cases much worse than their Wehrmacht counter parts.