The great advantage of vi is that it is included in the vendor-independent international standard POSIX.2 (IEEE 1003.2). On all POSIX.2 conforming systems vi is available. This allows users to move from one POSIX system to another without needing to learn a new editor.
The disadvantage of vi is that it is very cryptic and so not easy to learn. However there are people who, having mastered it, claim to like it. In addition, if you are likely to use multiple UNIX systems and to connect to them using different keyboards from different locations, then you may find it worthwhile to learn a "survival kit" of basic editing commands.