I don't know about 5G battery life, but with 4G the 6a outlasted all the other competitors in my household. Is poor battery life on 5G particular to Pixel phones?
There are many factors which determine why 5 G drains your battery. The most obvious being high speeds. Next being which frequency or band you are connected to.
Another is how much your radio modem is efficient in utilising your battery. There are others too but I won't go into the details.
One factor which sets pixel and other Samsung based dies is their radio modem is not that power efficient with the Tensor and other Exynos chip. So it might lead to more drain that other power efficient radio modems found on Mediatek, Qualcomm and other SoC manufacturers.
Again, this loss can be offset if you are connected to 700 Mhz which gives the best speed to power consumption ratio. But as mentioned there are many parameters at play when to comes to battery usage when using mobile data. Add to that, 6a has relative less battery capacity than what is in the market. But if Google releases updates for optimising the power consumption with 5G, you should see good battery life.
In my case, with 5000 mah battery, a couple of speed tests on Airtel 5G easily drained 1%. Now I already know Airtel doesn't have frequency in 700Mhz, so it will be inefficient.
Regardless of the technical jargon, see how the 6a performs on 5G, cause everyone's use case will be different. I don't need 5G for now so I stick with 4G. With time, you will be the best judge.