The LTC3119 is a very nice part and was initially on the list for consideration for this driver, but it's unfortunately extremely expensive as icpart pointed out.
For a reliable driver, I think it's important to look at the absolute limits of the system to ensure it stays within regulation, either via software or hardware. From a hardware perspective, the greatest stress would be when the cell is at highest load (greatest output) at low capacity or cold, with a high V_fwd LED - in that scenario, the driver would need to operate in boost mode. It's possible to design the flashlight as a system with a designated battery and LED so the driver runs only in buck mode, but then it'd make sense to just develop a buck-only driver, of which there seem to be many on sale with very good power capabilities. I wanted the lume1 driver to be fairly flexible in the choice of batteries and LEDs and to be able to operate well near the limits, which helped guide the design choices. As mentioned, if I was developing one for a larger flashlight, I'd go for external FETs. Maybe this will be done in the future for a suitable candidate... maybe a 21700 flashlight.
As a side note, I'm hoping to send out a small number proto drivers to forum members to test since I may have missed something and it would be good to have some fresh eyes to take a look ... Then once we have a stable design, I'll make a poll to see interest for a run of boards. I like the idea of using a Google Sheets - this will come later. Meanwhile, anyone who has experience with electronics, the Anduril FW, and is able to do their own measurements, EE debugging, firmware flashing and compilation, and would like to help beta test, do drop me a PM and we can work something out. Meanwhile, I'm in a process of doing some full characterization and comparison of the system.
Thanks everyone and stay safe in the emerging worldwide health situation!