If you are looking to become an English teacher in Japan or a teacher in Japan looking for some new perspectives, you will probably watching some videos on Youtube from other English teachers that are living in Japan. WARNING!! Do not watch videos like this.
The next series of videos are from a Youtuber called Tkyosam. This individual is somewhat well-known for his rants because most of his videos are rants. His major complaints are when he was an ALT (assistant language teacher) working for a dispatch company. Many of his complaints have to do with the nature of Japanese schools and their relations with their ALTs. I have been an ALT for about 7 years now. A lot of his complaints have some truth to them. However if you are working for a dispatch company, you will be screwed. I know from experience. The dispatch company in my opinion does not have your best interests. His other complaints have to deal with his appearance when he is at the school. Japanese people are very conservative when it comes to their appearance. If you want to be treated like a outsider, then dress like one. If you cannot accept the changes that the school and/or company, please find another job. You have been forewarned.
This video from kanadajin3 is interesting. She has some good points. However, take with a grain of salt.
Teaching English in Japan is not for everyone. Some people are well-suited for teaching, while, others will tend to complain about their jobs. If you are flexible and willing to take a chance, try it out. Also, it would be a good idea to look the various labor laws in Japan. Japanese teachers will take advantage of you because they assume that since you are foreigner, you would not know your rights. Prove them wrong. Take a look at the General Union. They have some good information on your rights in Japan. Remember: knowledge is power.