That's only if your opinions are wrong though. I share my opinions on Reddit all the time and personally I very rarely see people being against anything that I say, and when I do it is either because I was objectively wrong or because I was on a sub where everyone disagreed with me.

That last part is important too. A lot of subreddits have a high concentration of a particular group of people. in r/politics you will see nothing but leftists, in r/anime you will see nothing but weebs, in r/teenagers you will see nothing but self-loathing, immature morons who say that they have depression but don't actually have depression and who are unaware of the fact that depression is a serious condition that can only be diagnosed by a real doctor, and anyways, you get the idea.

Taking this into account, you should be able to realize where it is okay to share your opinions, either because the sub you are in has a lot of people who agree with you (echo chambers are no good though) or because the sub you are in has a lot of reasonable people who will argue in an objective and civilized way with you if they disagree with what you say. This subreddit and r/tf2 have a lot of reasonable people, as far as I have seen, and those are really the only subs where I hang out often, other than r/ShitPostCrusaders, where nobody is ever serious about anything.

In just about the entirety of reddit, being against "antivaxxers" is okay, but in subreddits specifically created by antivaxxers as echo chambers for other antivaxxers, you'll be downvoted to hell and back if you bring up any facts or science or anything like that.

Look, as long as your opinions have solid and reasonable bases and are backed up by facts, sharing your opinions should be fine in most subreddits.