This paper investigates the challenges for Resource Allocation (RA) that can allow reconfigurable systems to adapt the limited resources in a multi-sensor network during operational phase to changing missions, users and/or environments. Nodes can have multiple different radar front-ends, but also electro-optic and acoustic front-ends. Using a hierarchical model to explain what RA is in order to avoid ambiguity of terminology, this paper gives a novel insight into the RA challenges: the limited resources should be successfully allocated to the operational tasks while satisfying certain quality criteria and considering task importance. To understand the nature of the RA challenges, the resources and capabilities are discussed and basic principles for resource allocation are present. Although the benefits of a heterogeneous ad-hoc sensor network are clear, it is certain that allocating the resources to maximize mission success is a major challenge.