The problem is that leagues aren't as customisable as the online championship mode in 2018.
In leagues in 2019 you are unable to set the cars as 'realistic' performance. This is my main issue and means that you completely lose that 'roleplay' element of both driving for williams and seeing who can battle the midfield the best. Yes, you can make them F1 2019 cars, but it's forced to 'equal' performance and not changable like in unranked. Also, the general customisation is limited so can't have random / reverse grid orders or change the weather and time.
Also, though I've not personally tested, I don't believe you can save between sessions on a weekend in leagues (e.g. after quali and before the race) and if one leaves, or if you need to restart just the race because one of you made a serious error, you need to restart the full weekend, not just the race.
So basically our options in 2019 are
Leagues - forced 'equal' performance, very limited customisation for things like time / weather / random or reverse grid. Does however allow saving of progress and points as you progress.
Custom unranked - full customisation possible but no ability to save between races so points are lost between sessions. Yes you can of course use Excel, but this isn't an issue in 2018.