General Synonyms:
* Useless
* Futile
* Pointless
* Fruitless
* Unproductive
* Ineffective
* Powerless
* Worthless
More Specific Synonyms:
* Unimpressive: Describes something that fails to make a strong impact.
* Unsuccessful: Simply means not achieving the desired outcome.
* Weak: Suggests a lack of strength or force.
* Incompetent: Implies a lack of ability or skill.
* Impotent: Suggests a complete lack of power or influence.
* Inept: Indicates clumsiness or lack of skill.
Informal Synonyms:
* No good
* A waste of time
* No use
* Dead in the water
The best synonym for "ineffectual" will depend on the specific context and the meaning you want to convey.