The effects of NESTs’ instruction on Taiwanese students are worth investigation. A semi-structured interview study was adopted to collate in-depth qualitative data, along with a grounded theory to analyze the data. The participants included eight NESTs, currently teaching at the time of the survey, from four technological universities in Southern Taiwan, coupled with eight full-time non-NESTs and 12 English major students to gain multiple perspectives. The results demonstrated that NESTs’ instruction had both positive and negative effects on students’ EFL learning. The positive effects included: (1) NESTs’ language edge benefited students’ English learning, especially in conversational skills; (2) active teaching methods were characterized by the NESTs’ delivery, such as fun activities, rich body language, and interaction, which encouraged the students to undertake speaking and listening activities in the target language; (3) the students in NEST classrooms developed cultural awareness. The negative effects were as follows: the students’ language barrier, misunderstanding of the NESTs, inability to respond fluently in the NESTs’ EFL classes, and, most seriously, learning anxiety and stress. Gaps between the NESTs and the students in relation to culture and expectations also resulted in difficulties in real communication in class and sometimes tensions. Students’ bilingual and cross-cultural needs were at odds with the monolingualism or "English only" classroom policy of most of the NESTs.