I have Tangem 2 and use it primarily to top up my crypto card while on the go. Because of that, I would have little interest in their "shell" accessory which requires the wired connection. One of the key advantages of Tangem is that it connects wirelessly via NFC to its dedicated mobile app that is very convenient for mobile use.
Shell can be used 100% air-gapped. No wired connection. You sign transactions by pointing Shell's camera to a wallet like UniSat, BlueWallet, ... etc. Then confirm on Shell's display & sign. Then let UniSat read the signature QR on Shell's display.
Fully air-gapped. Shell's usb port can 1) have data turned off 2) be used for charging only.
Keycard also works via NFC - with one open source EVM wallet - Status.im - for now.
Tangem offers the unique capability to function as the seedless wallet utilizing an innovative backup technique. Each card in the related set is capable of restoring sensitive information such as your key and PIN. As Zack Herbert
CEO of Foundation mentioned
in the recent interview, SEED phrases may soon be obsolete. Based on the information from your website, it appears that Keycard still relies on seed phrases. Is that correct?
Shell allows for backup cards, too.
backup cards and then destroy your paper backup.
The main difference to Tangem is that Keycard allows you to add backup cards after the initial setup.
Mind that Tangem doesn't have a "Shell", but you need to use their close-source app. Which had a security vulnerability where imported private keys could be leaked