This study investigates unproctored assignment-based assessment implementation in an online teaching environment compared to on-site assessment. A mixed-method research approach was conducted with the participation of 284 English-major students, 6 teachers, and 4 experts at a university in Vietnam. Data collection instruments included a questionnaire, in-depth questions, observations, and interviews to examine stakeholders' evaluation of unproctored assignment-based online assessment compared with on-site assessment; differences in students' learning motivation; drawbacks in online assessment implementation; and how to facilitate students implementing online assessment effectively. The quantitative results show that despite the significance of unproctored assignment-based assessment, students gave higher evaluations for traditional assessment, particularly in terms of measuring knowledge, examining skills, and ensuring academic integrity. Online assessment has no different impact on student learning motivation compared to traditional assessment. The qualitative data indicate that various problems in unproctored final exams challenged the effectiveness of assessment practices, such as poor adaptability to the learning objectives; more risks in submission; risks to academic integrity without proctoring, such as cheating, plagiarism, collusion, fabrication, and subjective evaluation; and limited development of skills and practice. Implications for online teaching and assessment are recommended.