How serious is it to have pituitary tumors?

Varies: Pituitary tumors can be large and compress the visual nerves. It is common for larger tumors to bleed. Tumors can release high levels of hormones, which would affect normal hormonal balance. If you have a tumor, you should see a neurosurgeon, who will have input from endocrinologists, ophthalmologists, etc., to help work up the tumor and decide the best course of treatment.

Serious pituitary: "seriousness" is based on size, secretion of hormones, impingement on optic nerve, ability for easy removal, likelihood for malignancy etc...

It's all relative: Pituitary tumors are rarely malignant, tend to grow slowly, and often present before they are very big. Also, some tumors only require medical treatment. However, larger or symptomatic tumors require surgery, which is generally safe but does carry significant risk and requires a (usually short) stay in the ICU postop. Tumors can be safely removed, but they can be difficult to completely remove.