For document validation to allow multiple acceptable types for a field, one must nest a verbose $or expression in the validator like this:

{
    "$or" : [
        { "views" : { "$type" : 16 } },
        { "views" : { "$type" : 18 } }
    ]
}

It would be much easier to use $type if it could optionally take an array, which would be treated like $in and match any of the elements of the array.

The expression above would thus simplify to this:

{ "views" : { "$type" : [16, 18] } }

When such multi-type constraints have to be applied to many fields, validators would be much more succinct, making them easier to write (and read).