Formal:
* Unreliable: This emphasizes the person's inability to be depended on or to fulfill promises.
* Untrustworthy: This is a direct and simple synonym.
* Deceitful: This suggests a tendency to intentionally mislead or manipulate.
* Dishonest: This implies a lack of integrity and a willingness to lie.
Informal:
* Shady: This implies suspicious or questionable behavior.
* Shifty: This suggests someone who is evasive and unreliable.
* Two-faced: This describes someone who is deceitful and pretends to be friendly while being dishonest.
* Snake: This is a highly negative term, suggesting someone who is manipulative and untrustworthy.
Figurative:
* A wolf in sheep's clothing: This metaphor suggests someone who appears harmless but is actually dangerous.
* A ticking time bomb: This metaphor suggests someone who is likely to cause trouble or harm.
The best synonym for "not to be trusted" will depend on the specific context and the tone you want to convey.