I did a little searching, and here is what I learned:
Adverse effects on the lungs are not reported anywhere that I can find. It is widely prescribed for people with Cystic Fibrosis, and more recently Bronchiectasis. It works by drawing moisture into the lungs, thinning the sticky mucus which harbors bacteria so it can be coughed out.

It may cause temporary bronchospasm in some people with asthma. When it happened to me, I was advised to use it mixed 50/50 with normal saline (.9%) for a week, then try again at full strength - no further issues.

Hypertonic saline nebs are sometimes even used on infants under 2 years with bronchiolitis to help bring up mucus, so I think that speaks to the safety of use.

There has been some testing done to determine whether it acts on viruses. The study concluded that it didn't have any effect on the duration of viral illness.

There have been studies showing that the higher levels of hypertonic saline immobilize NTM in a culture medium. Specifically, all common types except M abscessus were reduced by hypertonic saline of 3 - 7%. I have not seen studies evaluating the effects on NTM in the lungs.

Hypertonic saline can irritate the throat, especially at first. Gargling with plain water after nebbing has worked for me.

Does this answer your question?