UNSUPPORTED_FEATURES

* does not work with :dependent => nullify because
  def nullify_has_many_dependencies(record, reflection_name, association_class, primary_key_name, dependent_conditions)
    association_class.update_all("#{primary_key_name} = NULL", dependent_conditions)
  end
  This does not trigger callbacks
* update_all, delete, update_counter, increment_counter, decrement_counter, counter_caches in general - counter caches are replaced by this gem, bear that in mind.
* attr_readonly - no technical obstacle, just not yet supported
* attributes before typecast behave unpredictably - hard to support
* ActiveRecord::Rollback is unsupported - the exception gets swallowed so there isn't an opportunity to rollback the cache transaction - not hard to support
* Named bind variables :conditions => ["name = :name", { :name => "37signals!" }] - not hard to support
* printf style binds: :conditions => ["name = '%s'", "37signals!"] - not too hard to support
* objects as attributes that are serialized. story.title = {:foo => :bar}; customer.balance = Money.new(...) - these could be coerced using Column#type_cast?

With a lot of these features the issue is not technical but performance. Every special case costs some overhead.