> I think the pi is not particularly power efficient.
While there might be efficiency gains available, RPIs use a very small amount of power compared to a laptop. Idling a RPI4 uses 2.8 watts[0], which not only rates favorably compared to most laptops, but is far below what my charging cell phone uses. Maxed out my RPI can only hit 15W, since that's the maximum power that my official USB C adapter can output.
Now that being said, it does depend on what kind of load you're expecting to handle. If you're constantly maxing out a RPI, it is probably more energy efficient to purchase a larger server than to just keep adding RPIs. But if you're staying well below the theoretical max of a RPI, it'll consume far less than a used laptop.
> I think the pi would benefit from a more robust filesytem layout such as an overlay filesystem to allow continuous writes to /var to go to ramdisk.
This is one of the biggest drawbacks of the RPI; microSD cards cannot handle a ton of writes without frying. I've settled on using log2ram to ensure that logs are only periodically flushed from RAM to the SD card, to extend the lifespan of my cards.