There is a simple rationale for the differences between standard pneumatic, pneumovacuum and pistonless pneumatic spearguns. It all has to do with the metal running surface used in, or on, the gun and the likelihood of it being damaged and consequently causing rubber seal wear and pressurized air/oil leaks.
Standard wet barrel pneumatic speargun
The standard pneumatic gun’s running surface is the inner barrel tube and being internal it is somewhat protected from the environment of sand and grit which is found on the bottom in most oceanic environments, so it rarely gets into the gun, especially if the gun floats after the shot. Provided the water is clean they can have minimal bore wear even with alloy inner barrels. Guns such as the “Sten” have lasted for decades in regular use.
Pistonless pneumatic speargun
The running surface on the pistonless pneumatic gun is the spear shaft itself and it moves through a stationary seal in the gun’s muzzle. The spear flies out and collides with the environment, so it is more likely to run into sand and rocks. As the smooth surface of the spear is critical the spears therefore need to be made of better stuff, such as stainless and spring stainless steel. Now when diving in rivers and lakes the water often contains sand, silt and grit in suspension and that may gain entrance into the gun’s inner barrel when the gun is being muzzled loaded. If it is a standard gun then water is sucked into the inner barrel bore taking anything else in with it. If that is an inevitability then it makes sense to do away with the inner barrel entirely and let the spear take its chances in the environment. It is much easier to inspect a spear for nicks and scratches, unlike an inner barrel bore. Therefore you have guns like the "Vlanik" and "Evolusion" to be used in turbid waters
Pneumovacuum “dry barrel” speargun
The pneumovacuum gun has two metal running surfaces, the inner barrel bore and the spear. So why use it? Well in turbid water you can use it to keep suspended material out of the inner barrel bore, as only a small volume is open at the muzzle entrance when the gun is discharged, so that is why you have pneumovacuum guns like the “Taimen” and “Pelengas”, but they have to use good quality spears.
I could talk about efficiency gains/losses of the various types, but as long as a gun is powerful and you take maybe a dozen or so shots on each dive outing then efficiency will not worry you too much unless the gun is extremely inefficient, such as found with a metal coil spring gun.