Thanks for the advice. Today I set up a few QB to test out what you said.
Demo charges work excellent. The big problem is maneuvering them into position to be effective. You have to decimate the entire smg companies on the first go, and make sure that when you are sneaking that you are not detected. If an aggressive Russian player does an advance order then you will be wiped out. If you get sound contacts while sneaking and the russian player does area target the next turn, all your troops will become pinned instantly and soon after they will rout.
I found FT's to be rather unreliable - they can usually get off one shot, but if any other squad spots them besides the squad being flamed, they are dead. They might be better off used as a defensive measure for the throwing demo charge squads should the enemy decide to storm you with hidden troops not affected by the mass explosions.
Another problem is cost for troops. Assuming no maneuvering difficulties as stated above, you need two pioneer platoons vs one SMG company. This is 468 vs 267 points. To reduce the cost you can choose mechanised pioneers, which come with 1 man less per squad but same number of demo charges, and you can also get rid of flamethrowers. This comes out to 272 vs 267 points. Not bad for troops that are also capable of fighting long range or destroying tanks.
I also gave flammpanzers a try. They can totally decimate a company of SMG's in seconds, and have an almost zero % chance of KO - SMG fire cannot penetrate the armor at all, and molotov coctails rarely do enough damage. Probably better to get rid of the FT's in your platoons and replace them with flammpanzers if the terrain suits it.
I will try 150mm next