Assuming that TLS will not be broken in transit is a common and reasonable assumption. The easiest way to break TLS is at the endpoints.
Implementation errors in your code, cert mis-issuance, errors in the underlying TLS implementation (certificate parsing and validation errors are quite common) and device compromise are all things to think about.
It might be useful to think about what is your responsibility versus the device vendor's responsibility versus the user's responsibility.
Comments
Assuming that TLS will not be broken in transit is a common and reasonable assumption. The easiest way to break TLS is at the endpoints.
Implementation errors in your code, cert mis-issuance, errors in the underlying TLS implementation (certificate parsing and validation errors are quite common) and device compromise are all things to think about.
It might be useful to think about what is your responsibility versus the device vendor's responsibility versus the user's responsibility.