The most obvious is that it’s not an illness that can be seen by the eyes
per-say. At first glance and maybe several, it is possibly never seen. Unless the symptoms manifest in other ways. We look like you, speak as you do. Love, hate,  hurt,  feel just like you. But the filter of emotions are not the same.

You can’t take an X-ray or MRI to find it.

In most cases its never diagnosed properly nor by the first evaluation. It may take several doctors, evaluations before getting properly diagnosed. This is frustrating for someone suffering with bipolar. Especially if given the wrong medicine. AntidepressantsIn most cases will induce mania.

Statistically speaking it has the highest suicide rate than any other mental illness. If left untreated, the repercussions can be detrimental both for the sufferer and those who encounter them. Between 30% to 50% of those diagnosed attempt suicide.

But personally speaking the biggest way that the illness can be serious is by the mast majority of ignorance on such matters. Not enough knowledge among society, those who treat it as though it is leprosy, or make threats against those who suffer from the illness, or use the term to diminish power of those who suffer from it.
Think about it… Many who are diagnosed feel ashamed or less than and won’t even acknowledge their illness around others. This is because lack of understanding on the subject, uneducated assumptions based on false information. Many people cause great damage by making fun of, or acting superior to the suffers of the illness.
These are the ways bipolarcan be serious. Educate Yourself, and your loved ones.