Maybe you could reduce the flamethrower’s special ability to attacks in the same hex, meaning range 0. I can see a flamethrower frying a tank in close combat, but maybe not from a range of 50 to 100 meters (the rule book states that each hex is 100 meters from side to opposite side).
Either way, the odds of throwing three sixes on seven dice are low.
Re: a house rule to account for a flame-thrower blowing up when hit by opposing fire, probably not necessary considering only 4 successes disrupts, 5 destroys it.
I think most house rules are stupid and unecessary anyway.