I find balancing to be easier in this version then others. The over the top wizard that was prevalent isn't really anymore, and the magic items have been toned down somewhat. So is the cleric that essentially negates all HP loss and mitigates conditions, like the pacifist in 4E.
The MC issue is handled easily, ask for story why and if they don't have one in about 30 seconds then its just PG as the only reason and disallow it. Another easy way that works in my group is XP cost that's paid, or working with a players absence for a session and his wanting to MC to put him behind the group in XP to simulate time away from game to enter new field. An enterprising way is for the group to train each other, then its just a strait XP fee. This also pushes a player to MC earlier, when it makes sense in a "career" as opposed to getting rid of a dead level.
The GW fighter builds are nothing, I played Dragon Age, where you just see hordes of scattershot, pinning archers as you moved up. I have never found them OP. Same with sharpshooter, they are good builds but a little terrain or even high winds can tone them down. All of these things can be worked with as long as you play monsters as their intelligence allows. A dragon that is the BBEG would certainly know most of every characters favorite attack routine so would avoid it.
Going the other way, I am DMing for the kids, age 10 and 8. Princes of the Apocalypse, and I have to play a wizard and a cleric since they have the fighter and rogue covered. I let them roll for abilities, the old standard of 4d6 drop lowest to make them a little stronger since we only have four and I read somewhere the game is built around groups of 5. The balance isn't an issue, being a little ahead such as 18 in main ability at start helps out a lot and that advantage has flattened out at current level of 4, where they can take a feat (more exciting) than an ASI. Also, using the point buy system never made sense, since it creates predictable builds.
I myself am playing a dwarf battlerager barbarian in another game. The dice rolls came up with 9 in intelligence and wisdom. Its great fun to occasionally force the group into a combat as talk over a perfect plan too long. Get inspiration every time also.