There are two way at looking at the level of ash vs engines.
As others have pointed we don't have enough information to tell the probability of ash combinations that do cause engine failure (not wear-out modes detectable as post maintenance issue).
But seeing as there is some particles of airborne ash throughout the world being ingested, we do have some meaningful data on what is safe to fly through. The more planes that fly through any measured level of ash, the more these datum become useful in at least raising the bar of acceptable risk.