I don’t think you can assess the effectiveness of volkite in a vacuum. Their inclusion in a list depends very much on the unit and the build for the army as a whole.
Let’s take a look at some examples:
Tactical support squad with Volkite chargers:
Tactical support squads can now take additional combat weapons, and the swap from flamer to volkite is free. So you effectively build a unit for close range support, that can also assault after shooting.
Tactical support with volkite calivers: fairly cheap mid to back field scoring unit. Given their 30” range and high strength, they will contribute more to a game than a objective camping tactical squad. Add a augery scanner to intercept units and push back infiltrators.
Heavy support squad with volkite culverins:
Good range moderate strength heavy 4 weapons, but adding preferred enemy makes this unit pretty brutal, ie via alpharius for the XXth or Autek Mor for the Xth. Granted not all legions have access to this, but a master of signal can help, and the new consul gives you master crafted.
Jetbikes with Culverins:
Jetbikes spend a lot of time jinking, so having a heavy 4 weapon will increase the probability of you hitting with those snap fire shots, for a moderate increase in cost over the stock heavy Bolter.
Medusa immortals with volkite chargers:
At two points per model, it would be rude not to! Forty volkite shots followed by 20 charging marines with FNP
led by Autek mor (fearless).....
Terminators with chargers:
I’m not sure the upgrade is worth the points here, terminators can shoot with their combi bolters and charge, so they gain no benefit from the assault rule. Also, terminators are primarily an assault unit, so every additional kill reduces the likelihood of a successful charge. I can’t tell you how many times my terminators have forgone their shooting to reduce the risk of a failed charge....
Contemptors with culverins:
Again, IMHO, not worth the points, which is a shame because they look awesome. There are better options for the points.
I’m sure there are plenty more units to evaluate, but the principals remain the same. You need to weigh the cost of the upgrade against the benefit that it brings to the unit.