Let’s wait that the flamer is nerfed to speak about the bolter 
More seriously, your last sentence is what is the problem. All weapons should be “fun” and be used in high difficulties.
Most of the melee weapons can be used in T4 / T5 difficulties. They can always have an utility.
But, ranged weapons, what do we see? Flamer, MG XII and bolter… sometimes one agri brauto.
And here, this means that there is a problem.
As I said above I don’t think MG XII is op, but MG IV and more than that the MG Ia are too bad. In fact lasgun have a weakness, the carapace.
However, Bolter is far away better than all weapons. Precise as is the revolver, full auto mode, 15 bullets magazine, piercing bullets, 95 bullets in your bag, no weakness… Revolver cannot compete, not also the shotgun. In fact the reload of a bolter is not too long. On a veteran you have a perk that makes you instant reloading your weapon. On a zealot, a ranged weapon is to shoot between two melee fights or when you encounter an enemy you need to kill at distance. Also the bolter can be used shot by shot and be really precise.
In fact the bolter needs also a nerf (perso, I would say a longer reload time). Once we will see people using all weapons, then the game will be balanced. Actually, there’s a big problem on ranged weapons.
However, I repeat what I said here… a nerf has to be a slight move… or you could make a good weapon an useless weapon.
If flamer (and here we return on topic) is nerfed, I hope that FS won’t go too far. Flamer needs to stay what it is. But, actually this is a too strong weapon, and that needs a correction.
I am sure that, even nerfed, flamer will stay a good weapon. And maybe, I would take more pleasure by using it than actually.