Whenever I see a thread on flying it seems to me that it's a clash of expectations rather than flight being the only problem. I agree on pretty much all of the points made by Erechel in the old part of the thread. In addition, I have played several flying characters and while they get some advantages there are many times when flying doesn't help. This was actually in Adventurer's League, shortly after they lifted the ban on flying races, and even with the limits on how a dm can modify things I almost never felt invulnerable. Part of it may be that despite the concerns about flying over 100 feet it takes several rounds to get there unless you're an aarakocra with a bonus action dash, as well as the very real possibility of visibility problems.
My current flying characters are a winged tiefling barbarian(level 3), a fairy land druid(level 4), an aarakocra paladin(level 5), an aasimar paladin with permanent wings(they were an option to take at level 5, she's level 7 now), an aasimar light cleric(same wings as before, level 11), and a winged tiefling archer of some sort(realized I wasn't happy with rogue, so I'm rebuilding him, he's level 12). Out of those only the last one even has the capability of flying that high and still being an effective fighter, so class and fighting style is a limit already. Only the first 3 had flight at level 1, since first they gave aasimar and tieflings special flight at level 5, then lifted the ban completely, but of those none have seemed too powerful. I will admit some old module adventures don't take flight at low levels into account, but those are fairly undertuned in general and I'm not aware of a similar problem with hardcover adventures.
In short, I don't think flight is that big a problem unless combat is in an area that gives the flyer a ton of room to move and the flyer is an archer. In exploration it makes things easier, but the only thing I can think of that can't be replicated with a familiar is the already iffy idea that a flying character can catch a climbing character before they fall. The one caveat to all this is that I personally stick with the party so my character can communicate with them and be in a position to help on round 1 rather than having to spend a round diving down. As for splitting the party, just send the familiar. They're a lot easier to replace if they get ambushed.