Let me revise that: for solid tumors, chemotherapy can cure people, but typically doesn't.
At one extreme, early stage (solid) cancers are primarily cured by surgery. At the other extreme (metastasis), chemotherapy is a terminal rollercoaster.
When you say adjuvant chemotherapy 'seems' to be useful in breast cancer, what exactly do you mean? How does a meta-analysis of 20,000 patients in randomized controlled trials showing a persistent reduction in risk of recurrence and death equate to 'seeming'?
At one extreme, early stage (solid) cancers are primarily cured by surgery. At the other extreme (metastasis), chemotherapy is a terminal rollercoaster.
In between those extremes (e.g. some limited local progression), adjuvant chemotherapy seems to be useful for breast cancer, but has only a very small effect for colon cancer (http://www.sciencedirect.com/science/article/pii/S0140673607...) and none for lung cancer (http://jnci.oxfordjournals.org/content/95/19/1453.short).