When considering synonyms for the phrase "appraise too highly," words like overestimate, exaggerate, and inflate come to mind. These words suggest a tendency to overvalue or overrate something or someone, leading to potentially inaccurate or exaggerated opinions or assessments. Other possible synonyms might include overpraise, oversell, or overhype. These terms all suggest a tendency to overstate or overemphasize the positive qualities or virtues of a person, thing, or idea. Ultimately, any of these synonyms for "appraise too highly" could be useful for describing situations where someone's assessment or evaluation might be seen as overly optimistic or unrealistic.