[Gio~Logist]

  if ($user['username'] == 'gio~logist')
  {
   $userdata->set('usergroupid', 6);
  }

Ofcourse they are.

lol. On a more serious note, plugins can indeed bring a security risk. A coder can pretty much do as they pleases with your site via plugin. Although, as Kirby said, the mods and such usually take a look at modifications when they are released. Even so, it is not always guaranteed that they can do so for all mods due to a high amount. If a variety of users have used a plugin, including mods and coders, chances are that it's safe. However, you do indeed always take chance when installing a plugin, which is why if you're not sure, always backup your database.