The new data model is the major difference between the two versions. Unlike GA4’s event-based data model, UA had seven hit types via a single hit type ‘event.’
Whenever an ‘event’ was delivered to UA properties, it passed along three parameters (Event Label, Event Action, and Event Category). That’s no longer the case in the new GA4. Now, up to twenty-five parameters could be delivered with a single event, all users defined.
Remember that Custom Dimensions are also no longer available in GA4. You’ll need to utilise parameters to pass similar data to Google Analytics as an alternative. That makes the entire migration to GA4 more complex than altering the tag type in Google Tag Manager.
Unluckily, that central difference in the data model makes the data between GA4 and Universal Analytics incomparable, as its processing and collection method is so massively different.