Luckily I got to experience invading the Inevitable City twice last night. I have to say I was very impressed. I may have to eat some humble pie. It is really fun.
I never knew that the Undercroft was actually part of the Inevitable City. For a second I thought I was in the Undercroft scenario.
It seems players are still learning what they are supposed to be doing. We rolled over Destruction all three stages both times in the PUG city siege, but I figure the fights will get more strategic as time wears on.
I am a pretty happy camper. I won my Sovereign Robe, a Sovereign ring, and ended up with 8 royal crests, and 3 warlord crests. I also gained about 20k+ renown each city siege.
It will definitely be worth doing even if I am locked out on the other rewards.