If everyone takes 5K IU/day, then you'd expect issues from about half of people (if baseline of 50% of people are low is correct). Those issues are nontrivial.
I personally think that most people (especially those who spend a lot of time indoors) should probably supplement, and most of the reaction is overstated.
It’s no big deal if you have sufficient magnesium, but for people who have low magnesium (which is common) it can make the problem worse.
This may explain the fatigue/lethargy at least one commenter here reported after taking fairly high-dose Vit. D supplements.
IMO this doesn’t mean you shouldn’t take them, but they should be taken with caution and ideally with clinical nutritional profile testing so magnesium or any other deficiencies can be addressed.