Formal:
* Insufficient: This is a direct synonym, meaning not enough.
* Inadequate: Another direct synonym, emphasizing a lack of suitability.
* Deficient: This implies a lack of something essential.
* Substandard: This implies falling below a certain standard.
* Unacceptable: This expresses strong disapproval.
Informal:
* Not good enough: This is a simple and direct way to express the meaning.
* Lackluster: This suggests a lack of brilliance or energy.
* Mediocre: This means average or ordinary, but not good.
* Poor: This is a strong word implying something is of low quality.
* Subpar: This is a common synonym, meaning below average.
The best synonym will depend on the specific context and the tone you want to convey.