Having worked in Oncology for the last several years, and some years previously, I know only of one person whose pain couldn't be controlled at the end but she had developed leptomeningeal carcimomatosis (malignant meningitis) from her metastatic breast cancer.
Pretty much always, death from cancer is peaceful and painless at the end.
On occasion the patient can become anxious and panicked if they know what is happening but, more often than not, they are unaware (with the help of a syringe driver).