Here is a quick article with some empirical proof of what we all suspected: Increased complexity of products under development leads to fewer concurrent development projects (Development Resource Planning: Complexity of Product Development and the Capacity to Launch New Products. Abraham Sin Oih Yu. 2010; Journal of Product Innovation Management):

The number of new product families that a firm can effectively undertake is bound by the complexity of its products or systems and the total amount of resources allocated to NPD. This study examines three manufacturing companies to verify the proposed model. The empirical results confirm the study’s initial hypothesis: The more complex the product family, the smaller the number of product families that are launched per unit of revenue. Several suggestions and implications for managing NPD resources are discussed, such as how this study’s model can establish an upper limit for the capacity to develop and launch new product families.