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:

• restaurants
• bands
• TV shows
• a class of investment

"Hit or miss" is not too formal or too casual; you can use it in most situations.