>Cancers are very patient-specific, and the main threat with immunotherapy is that the targeted cancer adapts quickly enough to escape the immune system again.

(I'm just an interested party and don't have any formal training on the subject.) I'm sure there are technical terms for it, but from what I've read the lack of regulatory features in the replication process of cancer cells tends to accumulate more mutations and genetic damage through each generation. PARP-inhibitors, for example, help fight cancer by suppressing DNA repair enzymes and letting the cancer cells get into non-viable states more often than healthy tissue.

In general though, this genetic entropy/volatility creates a scenario where adaptations can happen quite quickly.