Sombra and Genji were the two hardest heroes to make, probably.
Sombra worked very differently and had to go through major changes pretty late in her development (as I posted about earlier), and Genji started out having only his sword for a weapon (he is still refereed to as SwordNinja in our internal files, etc). His main kit initially involved invisibility, wall perching, and having an 'execute' animation where if he got behind you he could stun you and start this long animation which would just instantly kill you if he finished it.
The idea was he could easily pick off people who were not with their team, but otherwise it would be very difficult to use execute. This kit caused all kinds of problems though, especially his sword being his only form of attacking. Once we moved away from invisibility and instead chased the agile ninja fantasy, and made the sword his ultimate, everything started clicking into place.