This adds an additional attack surface. And if they make it open source like they claim finding a vulnerability in it might not be difficult, just takes an overflow somewhere.
Hard to implement ASLR well on a device so simple, afaik no one even bothers.

But this would only apply to a compromised system and many tricks are possible with such a system, such as moitoring wallet process memory and getting the private keys or downright making it send all your bitcoins to someone else.

If it comes down to it I'm sure someone will try to make the client give forged requests to the device when you legitimately ask it to sign something and I wonder how many people will read the small screen before hitting the accept button, not many I bet.

This device is security theater. Use an offline old laptop to sign everything. Nothing beats an airgap computer. This device interacts with an online computer directly... really bad theater.