Karmsun,
I am from the US and understand your concern. I think the reason you are not getting feedback is because there may not be many people in your situation in these forums. Most people in Montenegro use the local government run health insurance system. Meaning, they work / have a business and pay into the tax system which provides a government run health insurance program.
If you come as a retiree, I am assuming you will likely get residency based on buying a home here. So, I am not sure what options will be available. There are some private medical options. For example in Podgorica, there is a private hospital called CODRA Hospital which you can look into.
I don't know if there are private systems like in other countries where you can buy private insurance and have access to a decent network of providers. It is a very small country. And again, most people use the government run medical system.
About the local / government run system. In my personal experience, it is a small but functional system if you don't have major / complex needs. I had a routine surgery here and everything went fine. One caveat is the language. Though I asked about an English speaking doctor/surgeon, the reality is English proficiency is limited. In my case, I have been here long enough to speak an intermediate level of the language, so I managed Ok. The doctor's and nurses were patient and kind and adapted to my level of the language. But I am not sure how it would go if you don't know the language at all. I suppose you'd need to get a friend to go with you.
Before making a final decision, I think it would be best to visit the country and speak to some local providers. Maybe you can find out about private insurance options, and private clinics and hospitals like CODRA Hospital in Podgorica. I hope this helps a bit.