A comparison of Blackboard, Desire2Learn, Moodle, and SAKAI in light of their accessibility to all users, including those with disabilities. These LMSs were tested in eight categories including login, personalization, navigation, modules/tools, forms, authoring tools/content creation, help, and features affecting accessibility.
This blog post states that LMS's are great when they have some good features, but there is a topping off point. When they get too fancy, the user has difficulty in navigation. There is a "sweet spot" so to speak in functionality.
This blog post has an interesting reply within it for which the blog author gives a good response relating to the LMS serving as an obstacle to access course content. Learners can't get what they need quickly.
This post is related to private industry, but I think still applies in the case of education.