That is actually not true, NJ also attacks nests, lairs and citadels and if the player can time it than he can join the attack.
Here I was able to join a NJ attack on a lair: Nice combined Lair attack with New Jericho
It is funny and I’m also with you that we need more variety, but actually I would prefer simply lesser of these missions at all, regardless of haven defenses, nests, lairs or citadels. Over 100 missions for one campaign is a lot IMO 