They can, but the description of the Hellgun makes it seem like the RoF of the Starblasters with the same RoF may also be exceptionally high, depending on what 'sheets of lasfire' mean.
All we know about Fatecaster RoF is that they're noticably slower than a needler. There's nothing I can see to suggest that they're particularly fast, but we do know Starblasters are particularly fast.
I expect Fastcaster to be particularly effective again individual high value targets. They seem coded as super-sniper rifles to me, as opposed to the Starblaster super-battle/assault rifles.
One bullet one kill for enemies that rely on armour rather than omnidirectional energy shields, which is most, but not all. Also probably not against Necrons, who are made of amorphous liquid metal, like a T-1000 with less shapeshifting, so don't have weak spots.
Most foot-slogging infantry, and even most mechanized or armored infantry, do not use omni-directional energy shields. Even during the Great Crusades. Even Necron Warriors do not use omnidirectional shields. Their vehicles and canoptek constructs do, but those are not the majority of their forces.
Necrons are also not made of 'amorphous' liquid. Necodermis isnt an amorphous metal like the T-1000. It can remember its shape, and with the assistance of the necron machinery and technology can even pull itself together. But necrons have discrete parts and components inside them in order to function.
The only necrodermis construct that is homogenous are the C'Tan shells, and even they require the animating force of the C'Tan/C'Tan shards. Of them, the only one who can actually just 'will' stuff into being using raw necrodermis is the void dragon, who is not only a 'god', but the god of technology.
The fatecaster rifle is primarily an anti-infantry weapon, where its targets exist at scales where the projectile can actually do damage. Yes an omni-directional shield would probably thwart it for a time, but we are unlikely to encounter many forces that field something like that, and we would probably not want to throw a fatecaster, or at least a single fate caster, at such a target.
That is what developing specialist forces are for.
This is probably where we are talking at crosspurposes. My understanding of fatecaster weaponry is not as sniper weapons.
Its more like this.
View: https://www.youtube.com/watch?v=OsQQnYtSOXQ
Jesus christ why does no one use a smart link with their smart guns to take advantage of the autotargeting capability?
Anyway that is what I understand fatecaster weapons to be. They are an automatic gun that can fire what are essentially smart bullets that know exactly where they should hit you to do the most damage possible, until you are dead. Its not a sniper rifle. I am sure you could make a variant that is a sniper rifle in that it fires an insanely powerful projectile over insanely long range to do maximum damage. But the fatecaster technology just determines how the gun fires. It then gave us a variant for each potential form factor.
From pistols all the way up to naval-scale weaponry. Our sniper weapons are probably very specifically deliniated for us.