There was once intended to be a second, older Moonrise Tower in Baldur's Gate 3 that you'd also explore, but "the game was getting too big, and so we had to cut that out."
I think the pacing is just off once you get close to the end.
Once you get to Baldur's Gate, the game is a slog. If I didn't want to get the golden dice just so I could say that I got them, I probably wouldn't've finished the game.
The pacing when you get to act 3 is off. You've just made major progress in your personal quest, and discovered that there's a plot to take over the entire world, and you stop to help a kid find his mommy, and investigate a missing hooker? I had to ask for help on how to get into the city because I declined all of those quests because I felt like I had more important things to do. That said, the game is still a monumental achievement, and probably the best game I've ever played. You can't really fault it that it's still not as good the third time you play through it.
100 hours is too much for a game. Let's say I can play it 1-2 hours a day 5 days a week (I usually have stuff to do on the weekend), it'd take me over two months to finish it! There are other games I want to play!
I did finish it, but only because I had to spend over a month at home using up PTO before leaving my previous job for my current one.
Counterpoint: 100 hours is not too much, but it could be too much for you. That's fine, it's very obvious from the type of game it is that it's probably going to be 50+ hours long, and if you're not playing right at release, you can check Howlongtobeat.
I don't think playing a game for two months is a mark against it, as long as I like the game.
Not every game needs to be a short experience you can complete with a couple hours or needs to be finished within a few days/weeks. If I finished BG3 within a few days I would have thought it was a waste of money.
Seriously, a typical D&D session might last 6 hours and you accomplish nothing of note, but you have fun! Enjoyment should not be transactional with time.