Yes, that is right,
A given spear can only be driven so fast on an open track gun before it buckles on the way out, so there is an upper limit to how much energy can be applied to it by the gun and still be shooting straight. That energy can be provided by one band or multiple bands, the latter splitting the loading effort. Practically bands only get so large in diameter, so then you need at least two as few could load a single band with a 200 pound plus pull. Efficiency falls off with extra wishbones and band mass dragging through the water, but this can be offset by slightly stronger bands making up for it, however the same energy ends up in the shaft. To drive the shaft past this limit you can use a closed track gun, but this in turn gobbles up energy through extra friction in the closed track, so more band power is needed as you are throwing some away, but the spear energy goes up. Guns operating like this are often in an area of diminishing returns, more and more band energy is needed to gain just a little bit more at the shaft and the gun takes a long time to reload.
Spears can be driven at greater velocity out of tail end drive, closed track guns like hydropneumatics because a loading pump splits the loading effort, but these guns are nowhere as versatile to use as a band gun and take a long time to load. Pneumatics are similar, but limited by the loading effort, plus there is a danger you can bend the spear if you don’t push straight during muzzle loading. Usually this is done with one arm, you can use two, but you will be standing on something to do it and using a double-handed loader. If the spear gets away from you before latching then you are in big trouble.
All spearguns are limited by the drive length onto the shaft, a longer stock gun can drive the shaft for that much longer and a rollergun, like a pneumatic or hydropneumatic, can drive the shaft the full length of the gun stock, something a standard band gun cannot do. However the latter are the fastest guns to load and shoot, plus they enable a rapid change of busted bands, something that takes much longer on a rollergun. If you can toss one gun in the boat and grab another then that is not such a problem because you can always repair the first gun later after the dive.