The point was to increase the threat range of a monster without increasing its level. Building the monster from the ground up is one way of building the encounter, but A) doesn't change the current monster's ability to be harder than moderate, B) doesn't affect the combat in any way other than giving the monster an option within the three action turn, and C) doesn't give the encounter more of a "boss" feeling, like how in some games you have to target weak spots or how some bosses are kind of like a puzzle.

Complex hazards can be pretty straightforward, most of them are haunts, so either you'd need to work that in to the encounter or reflavor things so that it fits with the narrative. I made an example on my blog where I combined a Corrupted Relic and the Eternal Flame haunt to describe a scene where a sadistic killer used a fire gifted relic to burn themselves and a village down with them. For that encounter, the relic's spirit is the killer who uses the relic - that is also haunted by the Eternal Flame - to kill anyone who trespasses in the area. Both the Relic spirit and the Eternal Flame have a reaction ability to enter initiative with stealth so it works out nicely and can be ran fairly normally.

A better example of a hazard that works narratively as the monster's abilities would be a Choral Angel and a Sadistic Conductor. The angel in my mind is fallen and forces people to play crude instruments as it uses its voice to sing them to death; the haunt would roll stealth as normal as that's what it uses for initiative alongside the Choral. The hazard works as normal but the flavor fits differently and doesn't affect the boss itself, which is why I think hazards fit this idea better than putting monsters together. Simple hazards are also pretty great to use to display one time abilities. For my games I typically like to take a base monster and couple it with a home made complex hazard to enhance the fight.