I don't shy away from challenges, but at the same time, if a game forces you to spend a ton of time doing repetitive things just to level up needed gear for certain battles, I can't help but get bored along the way. I liked AC Origins for instance, but the repetitive grind to upgrade gear made it a game that was hard to come back to for another playthrough.
Ideally I like a game to reward the time you put into it to be good, by things like adding more and/or faster, more aggressive enemies on harder difficulty modes, and having enviro elements that change with the mode you're on too. For instance The Evil Within had faster, more aggressive enemies on Akumu mode, and disarming wall mines to use for upgrade scrap was nigh impossible due to a really fast timer and the need to stop it on a very narrow section of the dial.
It's games like these that make you innovate, like instead of disarming those mines, use distraction bottles to instead throw at them to blow them up when enemies are near after sprinting to attract them. You don't get the scrap, but you save ammo.
Lately I've been playing Jedi Survivor a lot. I like how the skill points you can find often have to be fought for in Legendary Enemy battles, or given a lot of thought as to how to get to them with platforming and enviro puzzles.
As for getting good ruining the fun, I would say no. Reason being, the kind of games that take a while to be good at also have enough in them to challenge you hard, and a lot of difficulty modes to progress through. Many also have added challenges if you opt to play New Game+ as well. In that sense these games also have a lot of replay value as they have a lot to offer and are less repetitive.
As for MP, which I don't play anymore as I'm 65, if getting good means it's too easy and boring, you probably just need to change the server you play on. In most MP games there's usually always someone that's better. Of course there IS a tipping point for ruined fun, it can be a fine line between too bored

, and too overwhelmed

.