I think the fear is that all this focus on the legacy of Roman culture will, intentionally or not, create implausibly stable outcomes in Western Europe. When it comes down to it, CK2 is not an exact simulation, and it simply isn't designed to effectively represent the nitty gritty details of ethnic and cultural identity. Without some allowance for the existence of (admittedly stereotyped) Gothic and Vandal cultures, you are going to end up with the absurd situation of a mod focused on "migration" which nonetheless has the Western Roman Empire lasting well into the first millennium. It's especially discouraging when the appearance of Islam, for instance, is so comparatively under discussed.
But when you look at a map of the 520s, what you see when you see "Frankish Kingdoms" or "Burgundian Kingdom" Or "Visigothic Kingdom," or "Vandal Kingdom" you're not seeing a kingdom ruled by Ethnically-Barbarian peoples quickly devolving from Roman urbanism to post-Roman serfdom. You're seeing Kingdoms ruled over by Romanized army officers of federate non-Roman origin who are advised by largely Roman public and ecclesiastical authorities and governing a self-conciously Roman population that demands Romanizing policies from its leaders. Think of Western Europe of the Middle Ages as a cooked meal. The recipe doesn't read "Add five parts Barbarian Migrations, five parts decaying decadent Roman civilization, stir." If anything it would read:
"1. Add Five Parts Romanized Army Officers of Non-Roman federate Origin, Five Parts Impoverished and War Ravaged (but still economically, intellectually, culturally, and religiously Vital) Roman Civilization.
2. Bring to a boil.
3. Drain off collapsing Western Roman Imperial Infrastructure.
4. Repeat Steps until Western Roman Imperial Infrastructure has been completely drained.
5. Add Five Parts Romanizing Political and Cultural Institutions to the resulting Post-Imperial Roman Kingdoms. Stir.
6. Add Three Parts Resurgent Roman Imperial Dominance And Aggression for a new, nutty flavor.
7. Add 1 Part Dust Veil Event. Wait 10 minutes while it rises.
8. Knead out famine in the mixture, spend 10 minutes doing this.
9. Add melted pnuemonic plague to the mixture and stir. Throw 40% of the mixture out. Let remaining mixture rise, then add a teaspon of bubonic plague every five minutes, mix, throw 20% of remaining mixture out, and repeat steps seventeen more times.
10. In the center of the mixture you will find - holding it all together - a largish clump of Pan-Mediterranean Roman trade in Roman goods. Take 50% of this and throw it out.
11. Add Lombard Invasion of Italy. Stir until mixture has darkened considerably.
12. Add Persian Invasion of Eastern Empire to mixture. Stir until mixture is thin and watery. Remove 15% more Pan-Mediterreanean Roman Trade in Roman Goods.
13. Add Loss of Imperial Domination in Western Europe to Mixture, and spice with Trade shortages That Lead to Localizing markets and New Legitimacy for Western Roman rulers (combination of Romanizing Origin Legends and
14. Add Starch of Heraclius to Mixture, let thicken for 5 minutes. Add back 10% of Pan-Mediterranean Roman Trade in Roman Goods.
15. Add Arab Invasion of Eastern Empire to mixture, stir until mixture is doughy. Remove 20% of Pan-Mediterranean Roman Trade in Roman Goods. (Left with 25%) Cut 1/4th of mixture out and throw it away.
16. Add Localizing Economic and Cultural Reorganizations to mixture, stir. Let rise for 10 minutes. At 10 minutes cut out another 10% and throw it away. Let Rise for 10 more minutes.
17. Divide remaining portion into 4 Portions. Make one portion 50% of the whole and call it Eastern Roman Empire. Make a second portion 15% of the whole and call it Hispania. Make 1 portion 20% of the whole and call it Gallia. Make 1 Portion 15% of the whole and call it Italian. Divide remaining Pan-Mediterranean Roman Trade in Roman Goods between the four sections.
18. Add Imperial Ambitions to Gallia. Rename it Francia. Add a measure of Eastern Imperial legitimization to Francia. Cut out 5% of its portion of Pan Mediterranean Roman Trade in Roman Goods every 10 minutes for half an hour, then set aside.
19. Reduce Eastern Imperial portion further by cutting 40% of it out, mixing those portions with long term Slavic and Arab conquests, and setting them aside. Add thematic system to the remaining original Eastern Roman Empire portion and mix. Set Aside.
20. Take the Hispania portion and cut 90% of it off, throwing it out. Leave 10%.
21. Cut 10% of Italia Portion off and throw it aside.
22. Take Francia portion and cut it apart. Cut it into many tiny slices, then look for any Merovingian blemishes and throw them away. Remove remaining Pan-Mediterranean Roman Trade from Hispania. Then add back into the bowl and mix in Carolingian reorganization. Add Italia portion and half of Hispania portion to Carolingian Francia, along with Saxon Flour, even more Imperial Ambitions, and a serving of Papal Support.
23. Go to the trash can and dig out 15% of the discarded part of the original mixture. Put in new bowl and label "Anglo-Saxons."
24. Divide Carolingian portion into thirds. Then recombine. Redivide. Recombine. Redivide. Continue doing this for a very long time. Eventually divide into three new bowls: Germany, France, and Italy.
25. Add Viking Invasions to France and Anglo-Saxon bowls. Remove 10% of Carolingian portion and mix with Norman settlement in Normandy until you reach a firm consistency, then add the new portion to Anglo-Saxon England and blend until even consistency is created. This will take a very long time.
26. Add back the Slavic and Arab held parts of Eastern Roman Empire to its original bowl. Blend.
27. Prepare new mixture called 'Seljuk Turks,' and prepare to add it to the Eastern Roman Empire.
The thing is that the Roman nature of the post-Imperial West was not a transitory thing. It was not fleeting. The Romans held on to their status as Romans for generations, for centuries. It was only through a combination of horrible plague, the devastating Persian and Arab invasions of the Eastern Empire, and finally the Carolingian reorganization of Francia that we get to the Middle Ages. Each was a major shock to the Roman system, and if we could mod those in as major events that would be awesome, but the end results of those events should not be assumed from the get go.