hit-and-miss
adjective uk/ˌhɪt.ənˈmɪs/ us/ˌhɪt.ənˈmɪs/ (also hit-or-miss)
没准的，碰运气的
If something is hit-and-miss, you cannot depend on it to be of good quality, on time, accurate, etc.
The trains are often late, so getting to work on time is a fairly hit-and-miss affair. 火车经常晚点，所以准时上班在很大程度上是件碰运气的事。