Personal opinions on hidden achievements?

Hmm... well, if you don't want to spoil the boss in question, you COULD have it so that you can see more spoiler-y achievements in-game, and so that you can ONLY see them once you are at a point in the game where it makes sense for you to know about the achievement in question. That way, players can learn about the achievements once it makes sense for them to know they exist, but until then, they won't be spoiled.

Does that sound like it might be a workable system?

Yea, I think this is for hiding spoiler content. For example, beating the game unlocks a hidden character. You might not want players to know about the character until they have it, but want to give them an achievement for unlocking it. But honestly, the internet is going to spoil hidden things for most people, especially if there is a community built around your game. Is there a way to see that you are missing a hidden achievement? In that scenario, it might just be a carrot for achievement hounds.

Hidden achievements seem best suited for really plot heavy events, you know, things that are huge spoilers for people who haven't played the game yet. Plot twists, final bosses, and the like.

I think hidden achievements are almost always a bad idea, for a number of reasons.

The last time I tried to get all achievements in a game was in Arkham Knight. Of the 10 or so I had left to complete, about 6 were hidden. This is despite having spent 132 hours in the game and having completed the game including all side missions and all DLC story missions. I looked up some guides to try and figure out what the hidden achievements were, but I never could figure out for sure what they all were. Most likely they involved getting to the end of the storyline (again) and beating the various bosses (again) in New Game Plus mode, which I couldn't be bothered with. But not knowing for sure meant that I didn't know whether or not my quest to get 100% was even feasible (what if one of the hidden achievements was for something far beyond my skill level to complete, like scoring 1,000,000,000 in mission x?)

Needless to say, these hidden achievements were annoying, and made a negative impact on my experience, which I wouldn't want to force onto my own players. The tricky thing in this situation is that the achievements probably were hidden to avoid spoilers (since otherwise a player could read them before they completed the normal game, before they even started new game plus). Though in my case, there was nothing left to spoil since I'd completed the normal game, so it was still annoying.

But probably the main problem with hidden achievements for me is that by making them hidden, I think it removes the fun, and the point. For me, the fun thing about achievements is seeing them as a goal and the horizon, striving towards them, and feeling a great sense of accomplishment when I finally reach them. You don't get that with hidden achievements.

I'd lean towards not having any achievements hidden, but if necessary couching them in vague/cryptic descriptions (eg. "Stumped: discover a mystery in the eastern forest", or "Defeat the Lost Guardian"). That at least gives the player a tantalising sense of what type of achievement it is (in your case, exploration-based: 'aha! I need to go exploring more somewhere around the East' or 'ooh! I'd better keep my eye out for a potential secret boss!') without giving too much away.