Consider the following:

  field :roles, :type => Array
  scope :admins, any_in('roles' => ['admin'])

it works as expected and produces the following criteria:
<pre>

<Mongoid::Criteria

selector: {"roles"=>{"$in"=>["admin"]}},
options: {},
class: User,
embedded: false>
</pre>

now, if I change the Array to Set, e.g. field :roles, :type => Set , then the produced criteria is wrong:

<pre>

<Mongoid::Criteria

selector: {"roles"=>{"$in"=>[[["admin"]]]}},
options: {},
class: User,
embedded: false>
</pre>