I had finished the game multiple times on Steam, then got it on GoG to make it easier to install Fallout London. Afterwards, I tried playing just the unmodded game, and I'm getting bugs that I have never gotten on Steam (across multiple playthroughs!).
This game really, strongly expects you to stick to the main storyline as much as possible. If you don't, you'll be wandering around a mostly empty world, occasionally punctuated by random encounters that are either way too easy, or way too hard.
After multiple attempts, I keep dying to an early-game encounter, even on easiest difficulty. I'm sure I'm just being bad, but I have no information that I can use to get better. This feels like only a mild improvement over a game that doesn't start at all.
The game starts out really strong, with lots to explore and mysteries to uncover. The late game doesn't live up to the built-up anticipation, though. I kept feeling like I was being asked to care about things I honestly didn't care about, over and over again. It made me appreciate the "stoic" dialogue options, which amounted to just asking the other party to get it over with. Full playthrough on normal difficulty took 44h 42m. That included scouring the entire open map, and finishing every single quest that wasn't blocked by my chosen course of action (or simply bugged). It was nice.
Every game that mixes tactical battles and a strategic overmap has the same core problem: if that tactical battle is challenging, you've failed at the strategy part, and vice versa. But at least within this narrow subgenre, AoWIII is still the best game available, even years after release.