It's an undemocratic process as it's not debated in Parliament and is usually drafted by unelected permanently employed civil servants and are often only rubber-stamped by the appropriate minister. Furthermore the Queen and the Privy council are unelected.
The large volume of delegated legislation also gives rise to criticism as it makes it difficult to discover what the present law is. This problem is aggravated by lack of publicity as most delegated legislation is made in private.
Lack of publicity