Out of curiosity, what's the reason for the antipathy towards the 1399 date? I barely touched EUIII.
Because it inevitably produced ahistorical outcomes every single time. I am not saying it is good or bad, but it just felt...weird.
For example: Golden Horde owned most of Russia at that point, and always stomped Muscowy. Hence Russia never ever formed. Then GH promptly fell behind in tech and was steamrolled by Bohemia (who was the HRE at that time) which resulted in ridiculous borders (you see EU3 had this mechanic that if you bordered a horde, you would automatically go to war with it every time after the truce expires between the two of you)
Another example: The Ottomans really underperformed; you would rarely see them threaten the Mamelukes or Hungary. Also Venice underperformed a lot, and was usually beaten by Milan.
Also in general, the historical unions formed very rarely (Spain, Russia, being the most obvious examples)
And one more - it took pretty long until you could get to colonization, so playing a colonial game required a lot of waiting initially. Also the colonists worked much differently than now
TLDR: 1399 produced really ahistorical outcomes every single time and history fans do not like that