Flamers are basically the lynchpin of SM early-game, because they have obscene damage, lots of roles, and almost no counterplay before T2 armor. They allow a good SM to shut down other racial infantry entirely while they prepare for their uber strong late game.
I understand the design decision to make them effective at nuking infantry, they are -extremely- good at it. Any infantry squad without defense upgrades will be destroyed by a flamer before it can even run away. This can stay, but only IF a way to properly interrupt and counter flamers is made available.
On top of this, flamers can both hit targets THROUGH heavy cover AND shield generators, the first of which is canonical but the second of which doesn't make any sense to me, rendering your own defensive structure a death trap and un-defendable. Not only this, they can fire up or down vertical terrain.
Their range is quite long, almost as long as the Tac squad bolters. This means there is very little risk for a squad to simply walk up in to range of an enemy infantry squad and turn on their flamer, knowing that there is nothing they can do once they turn it on.
And importantly, there is no counter-play, both on the micro and macro level. It costs (might be wrong about this) about 100 power to get a flamer squad out, an amount that is available at 00:00 seconds at the beginning of the game. This should be punishable with an early Falcon or squad of Nobz, but SM are compensated for this early power expenditure by Lascannon squads at T2, which cost no power and are very effective against both infantry and armor.
My main issue with the flamer is that it's not interruptable. Only knockback is able to prevent a flamer from firing (for a few seconds). Melee, silencing and blinding, all seem to do nothing. This makes a flamer squad almost completely immune from melee attack because of the damage dealing ability of the flamer. A flamer squad should be, by getting within flamer range of my troops, putting itself at risk of melee attack, whereas currently they are a hard counter to all early game melee infantry. Since they counter melee infantry, ranged units with equivalent range to Tac marines, and fortified units, I believe they are incredibly unbalanced at the beginning of the game.
My recommendation is to make flamers a channeled, interruptable ability, like Kill Team Ironmaw's flamer, such that I can prevent it from firing by tying the squad up in melee. This alone would create a viable counterplay to flamers.
I'd also reconsider allowing them to fire through Shield Generators, and up vertical terrain.