I've always had pretty good luck with two exceptions, AA and LA.

Not much problem with AA on domestic segments, but on int'l flights, they have a tendency to hold a lot of seats for full F until shortly before the day of departure. This makes it extremely difficult to plan an itinerary which guarantees you an AA seat in F on certain segments.

Also, LA can be difficult since some (all?) of their aircraft only have 5 first class seats.

At the end of the day, things usually end up working out on both carriers.