There's certainly variability, based I believe on the small sample sizes. A bad outing or three will have an outsized impact on a reliever's numbers.
But I do not believe that is the same thing as unreliable.
I believe in good telievers going forward over bad no matter if they had a bad game. And I DON'T believe in "just a guy" relievers no matter their streak of luck.
Relievers vary in quality just as much as starters do, And there are strong pens and weak ones, just as there are starting staffs.