In certain cases with the Fused, most notably the Shanay-im, we are seeing a phenomenon where they seem to be able to use their abilities without any seeming cost. I have an idea for how this may be happening.
Imagine that using a surge functions like a water wheel. Windrunner performs a lashing, and stormlight flows past the water wheel performing work. The entire time this happens, stormlight flows away and is depleted.
You would expect voidlight to work the same, and in some cases it seems to, like when Moash was lashed into the air to meet with Leshwi in OB. But this only appears to happen when the shanay-im lash something other than themselves.
When they lash themselves, the effect seems to be ongoing. I think that they've replaced the water wheel with a closed loop system. A feedback loop. Voidlight flows out over the "water wheel" and right back into the reservoir. So long as they don't push that power outside of themselves(lashing, slicking.,or placing an illusion on something other than themselves, possibly realmatic transition if they were to make a perp though I think Lezian is using the power somewhat differently here that relies in the Fused's spren like nature) or use it for the creation of matter, as healing does, there is no loss.
The downside to this, I think, is that the upper limit of how much voidlight they can carry is much lower, which means that again, using the shanay-im as an example, the maximum number of lashings they can use at once is much lower, giving Windrunners and Skybreakers a distinct advantage in the short term. And it makes the choice to lash something else or a need to heal much more relatively costly.
If a Fused can outlast a Radiant without the need for external uses or healing though? They can just keep on going.
I'm interested to see if I'm correct here. Regardless, their mechanics are intriguing.