To clarify, a guarantee of ethical treatment would have to be integral to the process. If we brought one or more back we’d be in a parental position, and like all parents we have a responsibility and duty of care to our children. This means guaranteeing their safety and well-being, particularly against the prejudices of the general population. This doesn’t mean keeping them isolated their entire lives, but you give presidents Secret Service detail for protection and I’ve not heard anyone complain about that.
The first generation would definitely be the most risky, since we know next to nothing nothing about things like Neanderthal psychology, and not really a great deal about their robust physiology, things like what diseases or disorders they might be prone to (at least in comparison to h. sapiens sapiens). We know that, as well as hunting related injuries and fractures, that they were somewhat prone to arthritis. Modern medicine, combined with examinations of a living being (not to sound cold, I just mean a medical check-up type scenario) could lead to them being more healthy than the average human.
We also think that their young matured quicker than that of our own, which means they’d probably need a specialised environment within which to learn, at least initially, as our own pedagogy based on human maturation may well go out the window with a Neanderthal child. These problems, thanks to information gained about them, would be greatly reduced after the first generation.
Like all parents we must also cut the umbilical cord at some point, despite the differences they are definitely intelligent. This would be the greatest discovery to be gained, their potential is completely unknown. What could they have accomplished if it was archaic humans that picked the short straw? We’ll never know. Bringing one back won’t answer that question, as it’s not Neanderthal progress or culture that (s)he has to build upon, but human progress and culture. Still, their biology and unique status would give said Neanderthal perspective we lack. It would ultimately end up adding to the collective body of human knowledge, even if Neanderthals turn out not to be too bright, that itself is an insight.
Regardless, as intelligent beings they must be allowed the freedom and self-determination we grant ourselves, or at least consider inalienable rights in most societies. How we deal with this may depend on what we’ve learned from their upbringing, if it turns out they are a potential danger to themselves or others there’s not much we can do but restrain them for their own safety. Still, the incredible things we can learn about them and us would make the process worthwhile.