に足(た)るTo be worthy of, To be enough, To be sufficient

Both assess worthiness, but with opposite conclusions and different tone. には当たらない is used to deny that an action or event reaches the level where judgment or response is needed, as in 批判するには当たらない ('It is not something worth criticizing.'). に足る means 'sufficient to deserve' and is a formal expression used to state that something meets the required standard for trust, evaluation, or action, as in 信頼に足る人物だ ('He is a person worthy of trust.').

Both judge whether something meets a required standard, but the evaluation is opposite. に足りない means ‘not sufficient to’ and is used to say that something does not reach the level needed for judgment, trust, or action, as in 証拠としては不十分で、判断に足りない ('The evidence is insufficient and not enough to make a judgment.'). に足る means ‘sufficient to’ or ‘worthy of,’ and is used to state that something does meet the required standard, as in 信頼に足る人物だ ('He is a person worthy of trust.').

Both judge whether something meets a standard, but the nuance differs. に恥じない means ‘not shameful'/'living up to’ a name, role, or expectation, focusing on dignity or appropriateness, as in 名に恥じない仕事をした ('He did work worthy of the name.'). に足る means ‘worthy of’ in a more objective sense, judging whether something sufficiently merits trust, evaluation, or action, as in 信頼に足る人物だ ('He is a person worthy of trust.').

While these two grammar points appear similar in form, they differ in origin and are used quite differently. たる primarily follows nouns and most often occurs in patterns such as リーダーたる者 'as a leader,' to express qualities or characteristics befitting someone in a certain role. In contrast に足る, can follow nouns or verbs to express the idea of being sufficient or worthy of something, e.g. 信頼に足る友人 'a trustworthy friend' or 語るに足りる発明 'an invention worthy of discussion'.