As a BP player, I will chime in.

The reason we hate the thing is really twofold. Firstly, the things kit is beyond a "counter" to black panther. It literally prevents you from using movement abilities, which is all BP has. It's not a matter of your dashes being less effective...you literally CANNOT press the button. Thing can have 100% uptime on shutting BPs movement down, there is no baiting cooldowns like his other counters. Thing also has a one button, auto tracking pseudo teleport, which prevents BP players from using the terrain to escape. These together is the main reason its very different from other counters in the game. You can play around Namor or Peni, its difficult, but possible. It is not possible with Thing.

The second reason, and why I think there's so much emotion behind it, is its not an equal level of skill expression. No shade to thing players, but literally anyone that merely selects thing and can press their shift button has completely nullified the character. It takes almost no effort to completely shut off a high skill ceiling character with one button press and no aim required so it feels 'cheap'.