Inconvenience: It's very difficult to find full-length games (at least 3 to 5 hours of game play). You have to go through pages of game jam submissions (despite the option to exclude them), someone's 2-minute first game, yet another flappy bird or some other popular clone, prototypes, wrongly tagged games and duplicates.
The site is flooded with games that take longer to download than to play.
The average session length is used differently. Some people will show average session as 2-3 minutes when they mean a level takes two to three minutes. You can't really accurately filter on "average session length" to get to games that will take a few hours to play.