It is dangerous, but not likely acutely dangerous.
It is possible for it to become accutely dangerous. More than a day in a row can lead to diabetic ketoacidosis, a dangerous and possibly deadly condition. We're talking usually days, but it can happen fast in some.
The biggest danger is complications, the longer term effects. There is no doubt that continued 400's will result in likely multiple of retinopathy , infection, amputation, nephropathy, heart disease, neuropathy, and possibly more. It is not medium term sustainable.