Diagnosed with severe graves 2 years ago and treated with methimazole and atenolol. Doing well now, but is it safe to continue with this drug treatmen?

Hyperthyroid Rx. There are risks associated with chronic Methimazole treatment--notably drops in your blood counts or severe hepatitis, which fortunately are not very common. The question to ask your doctor is whether your disease is likely to subside over time or continue indefinitely. You might want to consider either surgery or treatment with radioactive iodine for disease that is expected to last a long time.
Probably. Like any medication, there are potential side effects from methimazole, but most people tolerate it well, even for long periods of time. But you want to be on the lowest dose that works for you and you need close follow up.
Nothing is safe. All treatments have some risk associated with them. Not treating your illness is also a risk. Your doctor is there to balance the risk. Treatment varies during the course of your illness your doctor monitors this. There are possibly alternatives to your treatment if you aren't comfortable with the risks you are now taking. The alternative treatments have risks of their own. Ask your doctor.