Difficulty of normal content.

[Deslyxic]

Let me give a counter example of a fight that does all this but it's still easy, the Big Cheese (last boss of Lunar Subterrane), it does the conveyor belt things which is an exotic telegraph, and it does overlap it with other mechanics. The overlapping mechanics are a) standard orange circles, b) mines, c) half room casted attack. Of those three the most non standard are the mines but even if you haven't seen them before they are still red circles in the ground, easy "don't stand in bad" thing. At the ende of the day, it is an unconventional telegraph overlapped with pretty much conventional mechanics, and more importantly all is resolved at a very leisure pace, you will see the arrows in the conveyors long before it executes it.
Now compare that to the first boss of Alexandria (the morbol looking thing), it does a cone attack that has two variants but they have the same name in the cast bar (Immune Response, I believe), in their own are no problem since you have the visual telegraph. But then it overlaps it with the circles/crosses mech and now you need to watch which cone it is, you need to resolve the current symbol in the ground, you also need to be watching for the next symbols because it goes pretty fast, and also you need to be watching the cast bar to see which thing will resolve first. That's because the version of the cone attack is 50/50 and the combination of O/+ changes too, so sometimes you need to wait for the cones to go out before you can jump into a circle, sometimes is the opposite, it's not something you can rely on muscle memory to do. And more importantly, all this is happening very very fast.

Another thing about non standard telegraph is that it becomes mentally tiresome when they use them in quick succession like the second boss of Tender Valley, that by all right should be piss easy but because you need to be almost all the time be looking for the dunking bomb to see which lane will trigger on top of having to mentally visualize the approximate area of the others explosions, all this happening multiple times in quick succession. It gets pretty draining.
And then there are fights that I think are genuinely badly telegraphed like Valigarmarda that has two attack that hit more than 50% of the arena and are only telegraphed by the boss changing posture, then again, on it's own should be easy once you see them once but when it start to do them overlapped with other stuff, to the point it becomes hard to notice the thing even moved. I play with other players effects on low and it's still hard to notice.