Canonical (Ubuntu) - and therefore Mint - only provides support for the "main" and "restricted"* repositories and their respective "security" counterparts.
However, Mint and Ubuntu also have the "universe" and "multiverse" repos enabled by default. These repos, as well as any "partner" or ppa repo are NOT covered by Canonical's support and can therefore not be assumed to be safe. Their maintainers work purely on a "best effort" base (and the effort varies enormously).

Linux Mint Debian Edition (LMDE) is no alternative regarding security. In fact it's even worse. LMDE is based on Debian testing, which - as the name suggests - is not intended for use on productive systems**. To tame it LMDE copies snapshots of Debian testing to their own repositories whenever they think it works fine at the moment. Any security fixes Debian testing receives after the last LMDE snapshot will not be available in LMDE until the next snapshot, which means that LMDE is in a constant state of containing known but unfixed bugs.


*) As far as they can support restricted software at all (as in fixing security-critical bugs).
**) The fact that you might be experienced enough to tame testing doesn't change that intention.