Both games are their cores are well designed and fun to play, and obviously have some differences between them due to their developers. Blizzard games have a certain style about them, as do Valve games.
Also, the inherit problems with TF2 stems from the whole item fiasco, struggles to fully balance the game due to all of the weapons for classes etc, and while Overwatch has more characters, there are less weapons to balance since all heroes are limited to mainly one weapon, with some exceptions.
Overwatch's main flaw is the lack of substance - it has a lack of gamemodes, maps etc; which can make the actual gameplay get stale somewhat, which doesn't mean it's badly designed, its just not being utilized to its full potential.
I enjoy both, but I've been playing Overwatch a decent amount, and I will say I'd like it a lot more if there was more content like gamemodes and maps. Personally, I'd like to see more maps more than anything.