A bitcoin signer shouldn't have a full Linux stack, high speed lines and a user table.
More complexity => more holes.
Raspberry Pi is just a linux computer, like laptop. With a gigantic world of dependencies.
At the end of the day, all hardware is specialized. It's just a mater of scope. All parts of most HWW are available off the shelf.
I don't know much about hardware but my tinfoil is thick so I just use multisig with different devices and then ignore everything else about hardware 🐸
Buy one coldcard or buy multiple diy devices for same price and do multi-vendor, multi-sig. Which one is more secure for dollars spent?
Better yet don't put your whole stack in a single wallet?
Yawn
I think it depends. Having a device store keys could be a hole for some people. I don't think the potential holes matter so much if the device doesn't store keys or touch the web, assuming one doesn't want the device to store keys. But I do agree that it's obviously better to have the smallest possible attack surface even if you do want a RPi type signer. Most things can be better.