How serious is endometrial cancer in women?

Serious. Any cancer is a serious issue. The outcome depends on the stage and type of cancer. It is not feasible to provide a more informed answer without knowing the details of the type of tumor, stage of tumor and the general health of the patient.
All cancer-serious. But many endometrial cancers are caught early and are not only treatable but have a high cure rate. Fortunately, most endometrial cancers show up early with sign of abnormal bleeding most commonly bleeding after menopause. However, some cell types are more aggressive and some cancers are found when they are invaded or spread. Never ignore an endometrial cancer.
Endometrial cancer. It depends on the type of endometrial cancer, stage, grade, completeness of initial cancer.