railroading is worse

Battle Chasers: Nightwar
The Tolkas Arena battles are pain the ass because you are required to fight waves of enemies within a certain amount of time. The combat is turned-based, but the timer is real-time. That means time will keep on running whether you are navigating a menu or watching attack animation of enemies or your party. To get around it, you either have to grind a lot and/or speed up the animation. Unfortunately, playing through the arena is the key to getting some of the best gear in the game. It should be noted I finished the game not that long after it was released on GOG, so I don't know if this issue has been fixed.

Geneforge
Power Core and similar areas where just being there causes you to take constant damage. Also, there are some areas that spawn endless mobs. I end up going to those areas anyways, since I ran out of areas where I could explore and gain exp and loot.

Planescape: Torment
Modron Maze....maze-like levels and randomized to boot. Unfortunately, necessary to acquire certain party member.

In Horizon Zero's Dawn case of the sun marks and secret armor mission, I find the side quests to not ruin the game. In my opinion of course, if a side mission or puzzle rewards the player with high gear or something, I do not find it ruining the game. Another case, in mark of the ninja and don't starve, I like the (not really side missions) but tasks to get unlockables very enjoyable with getting new things to try out. Without microtransations to get characters or gear like DST for Wortox, I find it rewarding if done correctly as a pervious user mentioned about grinding riddles and long quests. With more content being able to enhance gameplay or change the style completely I find it intriguing. Of course others may disagree with my thoughts, but this is my opinion. Feel free to tell me what you think so I can see your thoughts.

I'd argue that it can ruin the game if either:
* The side event, or even just the reward for it, is permanently missable.
* The reward in question is a gamebreaker, is too easy to get, and the game doesn't warn you about it being a gamebreaker.
* The challenge needed to get that reward is not genre appropriate.

Interesting...because this is the kind of thing that appeals to me in a game. I guess I have roleplaying games in mind specifically. I'm not a big fan of the modern Bethesda approach where the player gets to be, simultaneously, the biggest baddest warrior, sneakiest rogue, and most knowledgeable sorcerer in all the land. It seems to me better for roleplaying games at least if there is missable content as it encourages more careful consideration and can keep playthroughs more narrowly focused (in addition to encouraging additional playthroughs to see more content by trying out different roles or choices).

I may be OK with it if it's a conscious choice that the player has to make (though being able to switch later would help).

What I am not OK with, however, is if simply progressing through the main story closes off side quests; I don't like it when the game arbitrary disallows the approach of "do main story first, then go back and do side quests that are missed". Particularly bad examples of missable treasures include one of the Blood Swords in Final Fantasy 2 (and it's the only one in GBA and later), some bard songs in Final Fantasy 5 (especially the one that raises magic power), and, in Paladin's Quest, both one of the Gigabl's (restores everyone's HP fully, 9 uses, pretty much required for the final boss) and the Heart spirit for Midia, as well as the Adult Cl.

Also, the length of the game matters; for an 8 hour game it's OK if the player has to make such irreversible choices, but in a 60 hour game it's not.

And you can't just CHOOSE not to use said gamebreaker afterwards. For example, nothing too wrong with the reward being an Infinity+1 sword if you can just stick it back in the inventory and keep using the normal gear. But if my reward is a passive skill that raises my attack to godlike levels or my HP regenerates faster than the enemies can hurt me, then I'll be surely pissed that I even got it.

@OP

Depends. I always want all of the story, all of the lore... so I always want to do as many side-quests as possible. If those are unreasonably hard or tedious it might really hamper the enjoyment of the game.

What doesn't faze me are optional mini-games or "hunt the 100 collectibles". I will pursue those as long as it's fun, and stop when it isn't.

And when important things are hidden behind them?

Then they are not "optional"?

Depends of course on what you deem important. When, for instance in Witcher 3 Zoltan asked me to help him out with Gwent-related things, I actually felt bad to let him down in my first play-through (when I didn't like Gwent, and thus couldn't help him). In later playthroughs I actually learned Gwent and built a good deck (which was fun for me after all, but maybe not for everybody), it let to hilarious side-quests and lots of fun. Not really "important", but still a little chunk of the game missing, if you don't do them.
Gladly, they offered a Gwent-difficulty-setting in a patch, so even people with no talent for it can enjoy the content - you still have to go through the motions though (play various people, build a deck) though.

Brings to mind the bizzare way Mysteries of the Sith handled skill points, by awarding them for finding secrets in levels (and only that). They still work like optional collectables, you don't have to find them all, technically you don't have to find any I guess, and yet they're the only way to improve your character. It's such a weird and meta game concept, to tie character progression to a vestige of old FPS level design entirely abstract to any story or level objectives or core gameplay.

Isn't that not really that different from the way Metroid handles it? (The only real difference is that, in Metroid, each power-up has a fixed effect; a missile expansion will always be a missile expansion, for example.)

I personally had no problem gambling in DQ11, I had tons of fun and won valuable items (mostly crafting ingredients) and I ignored the Casino as much as possible in Ni no Kuni.

Edit: in both cases, that certainly not an optional feature that would ruin a game.

That wasn't just Mysteries of the Sith, it was already like that in Jedi Knight without the expansion. Personally I like it as a concept, since imo it reinforces the gorgeous level design which is one of the main attractions of the game, makes you pay close attention to your surroundings. But it's admittedly pretty "gamey" and I can see how other people might regard it as annoying or even immersion-breaking.

Thankfully I don't have completionist tendencies, so this doesn't usually bug me. Like I'm playing Pillars 2 right now and am skipping 2 of the 3 DLCs and I never did a lot of the faction quests because I knew which faction I was gonna pick. Even some of my favorite games ever like Morrowind and New Vegas there are quests I've never done. I think it's nice to leave things for future playthroughs, and also not to burn out on the game.