On the Vitamins section of the page, it says "There is a limit of 10 per stat, however they only work on the first 100 EVs." but in the recent games since generation 8 the limit has been removed and lets you increase EVs beyond 100.
And on the Power item section, it says that each item gives you 4 EVs in the specific stat the item gives you, this was changed to 8 EVs since generation 7.
