A joint inspection team from Ofsted and health watchdog the Care Quality Commission, found that services in the area have made too little progress in implementing SEND reforms since 2014.
Although leaders were acknowledged as having "ambition" for children and young people with SEND, inspectors said they had failed to translate this into a clear and coherent strategy for improvement.
"The outcomes achieved by children and young people who have SEND vary too much," a letter outlining the findings states.
"This is especially the case for those in mainstream secondary schools and young people who have SEND aged 16 to 25. There are gaps in the local area's analysis of the outcomes achieved by children and young people who have SEN and/or disabilities.
"Local area leaders do not have a well-thought-out strategy for developing the knowledge and skills of frontline practitioners or a coherent approach to developing and implementing effective, and widely understood, systems and procedures."
Inspectors added that local leaders have a superficial understanding of the needs and experience of children and young people with SEND and that arrangements for jointly planning and commissioning services in a needs-based way are "undeveloped".
Meanwhile, co-production working - where children and young people, families and those that provide services work together to make a decision or create a service which works for them all - was found to be not sufficiently embedded in the local area's approach.
"Frontline staff work hard, individually and in their teams, and make a valued difference to children and young people who have SEND and their families," the letter states.
"Crucially, however, families have starkly contrasting experiences of the local area's arrangements for identifying, assessing and meeting the needs of this group of children and young people."