Royal Engineers dont have extra line of sight or flamethrowers and while they have the anvil upgrade thats locked behind tech
Pioneers have their line of sight and flamethrowers which is used and is effective through out the entire game
Royal Engineers while a good unit dont have as much utility as pioneers in my opinion and the utility they do have can also be fulfilled by other infantry units of the UKF kind of like Rear Echelon and even Rear Echelon can get flamer throwers
and while the wasp on the Bren Carrier does exist that thing tends to have a short life span
Royal Engineers are probably only useful when you get triple lmgs which will cost you 190 munitions but you can also give those lmgs to your Infantry Sections kinda like Rear Echelon you can give them bars or give it to Riflemen the choice is up to you
Aside from LoS, Sappers are better. 210MP, best repair, very good damage. I would take sappers' stens over pions MP40s any day. Sappers are also in a faction that doesn't have to build stuff so they are basically front line units. I'm not saying they are OP or anything, just saying they are the best "engineer" unit in my opinion.