Previous large trials. Over what period of time? Define 'large' It would be even more interesting to find out who funded those trials. Who has the money for that? Ummm, let's have guess. The funding of drug trials is a deep and murky issue. The companies who want them approved seem to be tasked with demonstrating they are fit for purpose which sounds reasonable until you consider things like thalidomide.
I'm also guessing that the people who ran the large trial were not the ones reporting a 30%-70% early drop out rate. Define early, what does that mean? 1 year? 5 years? Who is reporting these drop outs? I'm betting it wouldn't be my oncologist if I choose to stop. Speculation is pointless, but let's assume those figures are correct, there would be a large number of people who persist with treatment despite suffering significant side effects.
The choice is clear, take the drug or don't take the drug. That the information about the product is confusing and inconsistent doesn't change the fact that it is currently (apparently) the best chance many people have of surviving the disease. Very frustrating, but it all comes down to your personal motivation--how much you are willing to put up with to achieve a poorly defined result.