From my understanding of the physics, all of the advantages of a roller gun come from where the top band stops applying force.
On a classic configuration the bands no longer produce force once they have reached their relaxed size. On a gun with 100 cm from band hole to mech this is somewhere between 20 and 30 cm from the band holes. From this point forward the spear is no longer accelerating.
On a roller gun the band pulls all the way to the tip of the gun. This provides two advantages. First, if comparable bands are used in both guns then the shaft from the roller gun will leave the speargun at a higher velocity. Assuming the mass of the spears is the same this will give it more kinetic energy/momentum and will lead to better penetration (also assuming the tips of the spears are identical).
The second advantage of the longer band stretch is the recoil. One way to look at recoil is wight the impulse momentum theorem. F = (Change in Momentum/Change in time). Because the spear is accelerated over a larger period of time the felt force is smaller. This is why guns rigged with 3/4" bands are described as "punchy" or "snappy" The band produces a large amount of force in a very short time. Guns with short thinner bands are often described as having a much smoother recoil.
Personally I think rollers have their purpose but are not necessarily for everything/everyone. They make sense for blue water where you aren't taking a lot of shots and reloading a bunch and the shorter length is helpful for maneuverability. They are also good for holes as mentioned. Perhaps their best use is in really dirty water shooting big fish. When you can't see more than a few feet having a 120 just isn't practical but you still need the power to penetrate a large fish like a striped bass. For SFL with clear water and constantly hoping in and out of boats I don't want to deal with the reloading process so I don't use them.
Just my .02