Unless I'm mistaken, a malicious package with the same name as a critical system package could be added to that untrusted repository by an attacker who had compromised it, and if the version number is higher than that of the 'official' package and your Apt priority config prioritises it, your system would download and install it without any verification.
This is a fairly niche attack vector that may not be considered a significant risk in some environments, but it's one to consider when establishing your threat model and risk appetite.
As a side note, it's worth noting that GPG signing packages is not a silver bullet either, especially if the signing keys and administrative access to the repository fall within the same security boundary (e.g. developer doing both from their PC without any segregation/sandboxing). However, it has proven to be a robust method so far, and definitely beats explicit [trusted=yes].