Some stuff can be destroyed, but not everything. Any quests you can lose completely are probably “deliver this note and get a small amount of money and XP” type quests.

I realize that making comparisons to earlier versions is questionable, but the Avernum remakes are generally more forgiving than the original Exile games, and I started playing Exile III without playing the first two, or any other games with this type of complex party building ever before in my life. I had no clue what I was doing, and an entire town got destroyed before I figured out how to play the game right. And I was still able to complete the demo (2 of the 5 main quests, or 1 of the 3, if you’re counting pairs of “either-or-are necessary, but you can do both if you want” as one quest and not two) just fine.

Also, there is a major quest that only appears based on a timer, but since you rest to heal in Exile (which takes time) but not in Avernum, and they didn’t change the time table, it turns out that most people beat Avernum III without ever seeing this quest. If they kept the entire game on the old Exile timer, and not just that quest, then it’s probably going to take a really long time for stuff to get destroyed.

If you ignore this quest when it comes up you get an automatic GAME OVER after a few days (I don’t remember how long, it will be enough time to respond). So that’s the only way to lose by taking too long.
You’ll get a message that you need to do it, and then teleport somewhere. I think once you start the dungeon the timer ends, so it’s not a big deal. You can’t back out without completing it once it starts. I can tell you what it is if you want the spoiler. Realistically, you’ll probably just naturally finish the game soon enouth and miss it :(

I didn’t do the timed quest when I played Avernum III but I remembered it from Exile III and looked it up. When I replay it I’ll find a way to eat up a bunch of time at the end so I can play it. I read that traveling back and forth on a ferry works.