When something is good sometimes, but not-so-good other times, you can describe it as "hit-or-miss":
A: How's the food there?
B: It's a bit hit-or-miss, to be honest.
Some things that you can describe as "hit-or-miss" include:
- TV shows
- a class of investment
"Hit or miss" is not too formal or too casual; you can use it in most situations.