[Coriolis illusion] can produce an overpowering sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down a hillside.
Oh, wow, ok, that sounds strong indeed... I'm starting to understand it would be super hard, while vividly feeling like I'm rolling down a hillside, to try and tell myself: "oh, I just need to focus on the instruments, and trust them..."
It's hard to imagine just how strong the feelings are from our current position of at home and typing on phone or computer, but it really genuinely is your entire body completely convinced one thing is happening, while in fact something else is happening.
(Note this is part of the reason why even with full visibility so many people get nauseous - subconsciously what your body thinks is happening and what your eyes tell you is aftually happening, disagree - sometimes strongly)
Moreover, anybody who can fly a plane should intellectually know how to read an attitude indicator. But it takes a lot of training to be able to trust that indicator, when you have powerful sensory evidence that makes you instinctively feel the aircraft is doing something that it's not.
See also: https://en.wikipedia.org/wiki/Sensory_illusions_in_aviation and note the long list of accidents at the bottom.