Hope this is in the right section.
I've already written a lengthy explanation on Reddit. It's currently on the front page of the /r/totalwar and has a lot of people agreeing with what i have said. https://www.reddit.com/r/totalwar/comments/968yot/legendary_30_loyalty_penalty_makes_managing/
My suggestion would be to keep the old loyalty penalties from difficulty. E.g. -20 for Legendary. It would still be challenging, but not ridiculous, like it currently is. (especially in the Rise of the republic campaign).