As previously mentioned, I've argued the unpopular side of many arguments, and I've found HN to be eminently civil as far as Internet forums go. However, my standard is "civility" or "good faith" and not "people agree with me". I agree, however, that many of HN's critics share the latter standard (and thus are happier on far more toxic websites), but I think it's an awful standard if it can be called a standard at all.
> This is not about civility but about whether or not the environment is structurally hostile to certain philosophical discussions
It's not hostile to certain philosophical discussions. We've had many robust political and technological debates (including whether the wealthiest should pay more tax and so on), but that doesn't mean the unpopular view carries the day (indeed, my view point on this is probably the unpopular one on this forum, though not by very much).
Now speaking to your concern about which viewpoints get more upvotes, in my experience as a frequent dissenter and devil's advocate, I've found that you can do a lot more on this site to mitigate the unpopularity of your opinions by demonstrating good faith, showing that you understand counterarguments, and communicating clearly. If you make everything hostile and tribal, you're going to have a bad time, which I think is a feature of this site (and which is also very likely the motivation behind so many of the HN=bad types of comments).