My worry is being chased back into the base as Soviets within the opening minutes of the game and being forced to A) choose a doctrine with guards or B) build T2. I'd caution Relic on this change but hope that the testing proves me wrong.
Even with both flamethrowers attacking the same target it does less damage than the current KV-8. Not much less, but still less (8.889 for KV-8 vs 8.572 for FHT, 4.286 per flamethrower.)
How to kill it? It's a 320 HP light vehicle. An AT grenade does 100 damage along with an engine critical, a ZiS shot does 160 damage, a mine does 200 damage along with an engine critical, I'll let you do the math on what would finish it off. It also gets penetrated by small arms fire fairly easily.
Regardless of your build, you have reasonable counters available to you as Soviets. You only might be in trouble if you go double sniper/heavy T1 and neglect building any AT, in which case you deserve it because double sniper is so boring to play against
The reason it was so popular is because it provided a mobile reinforce point that also deals a solid amount of damage to infantry, and with old veterancy was much harder to kill. It really strengthens the Wehr player's presence on the field. If it becomes too meta, an easy fix would be removing the reinforcement capability when it's upgraded with flamers, which forces players to choose what they want out of it. However, I think with its current stats it should be fine the way it is.