Living in Korea, particularly in the Seoul metro area, is safe and easy (if you can handle being away from home). In the aggregate, Korean children are well-behaved and motivated.
This leaves company and community support as the critical elements to consider. Avalon is a major company in the private education market, and they behave accordingly. You will be paid on time, have insurance, and generally have all promises delivered on. In the classroom, this large company mentality can turn into a bit of a widget factory mentality - focus on testing can be frustrating. But this is apparently endemic to Korean education in general. Materials were useful for the most part, but with most Avalon books you will have to improvise to keep the class engaged.
The community at my particular campus included a large contingent of foreign teachers, among which were excellent head teachers. You may want an experience where you are the only foreigner around; large urban campuses will not provide that experience. Personally, having many other foreigners available at various points in their ESL experience was invaluable and made the transition that much easier. The Korean teachers at Pyeongchon were very friendly and helpful to the foreign staff with only a few sparing exceptions. You will read of unfriendly Korean co-workers in your research, and perhaps they are accurate, but I am of the opinion that this is due to lack of effort on the part of the foreign teacher in question. If you make an effort to abide by Korean cultural mores - some of which can be a tad frustrating - and don't behave like a fraternity pledge during rush week, Koreans are generally very friendly people.
All in all, the company is recommended for their professional behavior and institutional support. If you are seeking to be out on your own with no hook to wester culture, however, a large urban campus is not for you.