dont forget also, that the fire nation has air ships, tanks, battleships, ect. and the earth nation has its tanks also. the death eaters couldnt insta rape with avada kedavera and would have to take time to deal with the vaious machineries
only person that could possibly cause problem to voldemort is an avatar, or someone who manages to survive against him. And there is literally four or five character in these two nations which can, on a lucky day, do so. The rest die pretty badly. Putting these people against whole legion of death eaters is slaughter.
the sheer number of soldiers in the fire nation and earth nation's army would wreck the death eaters. How many death eaters are there? lets say 200 (please correct me if im wrong, because i actually dont know). now, the earth nation army (who's weapon is the entire earth) would pose problems to all death eaters on the ground. And using a giant fire worm strategy wouldnt work because a) all earth benders can go underground/surround themselves with earth to use as a shield. b) their tanks have been shown to withstand fire from firebenders with no problem, c) firebenders could easily redirect the fire snake because its made of fire. now, if all the death eaters were flying, that is where the fire and earth nation's other military forces come in. in their metal sealed air ships and battle ships, fire benders can shoot streams of fire/ bomb the death eaters. and while yes, the death eaters would be able to dodge/redirect attacks/destroy many people and air ships, the sheer number of people would be too much for the death eaters to handle. the earth benders really do turn the tides because they can make a thick layer of rock armor to block avada kedavera, sink and crush death eaters into the earth, and much more if we bring sand and possibly metal bending into the equation. not to mention the lightning the fire benders can produce.