Discussjo about Viper current state

Viper is one of my favorite and most successful heroes yet according to Dotabuff Viper is at 47% winrate.

The hero lacks mobility and burst, but he is quite tanky, deals good damage, and is very opressive against some midlaners.

The hero at first glance doesn't look bad in concept. So what is preventing him from having a good winrate on both low and high MMR?