Unfortunately the problem is that what counts as challenging and complex tends to be subjective. For example; for me, personally, the boss that kills me more than any other is Adrammelech. Despite just being a series of re-used mechanics cranked up to eleven, I can't wrap my head around it and die all the time. I find that specific boss more complicated and difficult than any other raid boss or trial I've done. But most people who hear that will laugh and call me a casual baby for not being able to sleep walk through it - despite my favorite encounters being some of the ones that other people hate or find too convoluted and complex. So when it comes to a poll that doesn't go into too much detail about what it means, you're not going to be getting detail about whether the people saying yes are the people who want more bosses puzzle-like encounters, more trials like Seat of Sacrifice or if maybe they're people who find Leviathan complex and difficult and don't want more things like that because they still struggle to figure out the logic behind where he pops out on the sides of the raft.