Challenges and Solutions for Installing Ubuntu on NAS Devices

Installing ubuntu onto most regular NAS is unlikely to be a realistic option - they generally come with quite poorly performing processors that are better suited to running a use-case-specific OS, but...

i'd expect you'd be able to do it in the installer during hard drive config, but last time i did i think i remember using mdadm, that was post-install...

You can run any distro from a raid the only restriction is that the boot partition cannot be in "stripe" raid. Or the bios wont be able to find the needed info, as the bios is only able to read one partition at a time.

some distros will require you to create the raid setup manually, from a terminal, before running the automated install process.

the better option though, at least in my books, would be to setup the raid config in BIOS... that way any issues with the OS itself still leave you with your data in tact (plus the entire OS itself can be covered, traditionally you'd need to have the bootloader without RAID)

a personal/anecdotal recommendation would be to look into OSes that are a bit more fit for purpose, something like unraid or proxmox... they'll allow you to configure raid arrays very easily, but more importantly you then have more flexibility in running applications through their docker integrations and whatnot... plus, you can just slap a VM on the thing if you want an ubuntu shell. this becomes more complex if you're using a regular NAS, which would be (as above) relatively underpowered