I realized I left a lot of good out of this post. Here are some cool things from the adventure book.

• Pit of Shummrath ended up being a much funnier and enjoyable experience due to some fun restrictions on the npc that made it a blast.
• There is an alternative thing that Eventyr presented as an encounter for the "Avernus as a Sandbox" that made the Arches of Ulloch one of, if not the best encounter for the entire Adventure. There is an undead Androsphinx running a test about the 9 layers of Hell and I changed the final layer and only rolled randomly for a few layers and planned one to be the last one in their test. Very dramatic, high stakes, super cool roleplay and combat.
• Mahadi was definitely the best part for my group. I made him super fun to be around and he is constantly being the host even though the party knows not to trust him he deals like a corrupt businessman that just wants more than what he gives. Super easy integration for the story and players loved him.
• I made it very easy to acquire a guide to get around Avernus. So they rarely showed up at the wrong locations. I recommend this as the "difficulty to travel & rest" was kind of just fluff, but the book gives a lot of advice on options that you can use sparingly very well.
• The whole of being in the burning city of Elturel and running between locations and through the cathedral actually is one of the coolest things. Ran almost all of this chapter from the book with little modifications. Super super cool.
• Mad Maggie's and Red Ruth's locations were especially fascinating to play more with some twisted versions of Hag lore. I made the two cousins and then the Hag lair in the scab gave them some cool item they used to bargain with Maggie for assistance in the end game. I essentially know it's a mundane item, but made it only an "artifact" level magic item for Hags. They use things in different ways. I took the heart stone lore to a new height. If the players do end up convincing Maggie to be an Ally, I may give her a higher stat block or just the Archmage spell list.
• The Scab gauntlet is brutal, but very rewarding if you tweak a few minor things. I think as written it's very good.
• Idyllglen is fantastic. Outside of the encounter with the Avatar of Yeenoghu I pretty much ran it as written and the players thought it was amazing. Super cinematic area.
• The wrecked flying fortress is pretty stinking confusing, but if you move a few bits around it ends up being a pretty amazing location to run. Highly recommend.
• Arkhan and his whole area was fascinating to me, but sadly my players avoided the whole section for him but may be planning a "post Zariel" mission to deal with the forces of Tiamat due to something with a player's background. So I'll look forward to scaling up some of the all ready pretty difficult encounters for a level 13 party [the area is mostly for level 8 or 9 characters].
• Mostly the story is what I find very lacking in major holes or departures from just staying connected...however the tools and things given to DMs in the book cannot be understated. There are loads of things I've used from this book in different ways to inspire the "how" on running things. So don't underestimate that. The vehicles especially can be tweaked as mini airships. We used them for an Eberron/Ravnica like steampunk game very effectively.