IoT deployments in a variety of applications are characterized by the ubiquitous presence of devices for sensing, compute, communication and actuation. Mobile platforms (smartphones, rovers, drones) weave through already deployed (insitu) components adding new opportunities for data collection and processing. When deployed in mission-critical settings, IoT applications must satisfy multiple concurrent requirements (timeliness, dependability, interoperability etc.) - albeit with increasing complexity in terms of control and communication. In this article, we discuss how composability criteria have an influence across the IoT lifecycle - from design time abstractions and planning to on-the-fly adaptation to meet performance goals. Drawing from our experiences in high-stakes applications, we highlight key challenges and discuss building blocks towards the design of flexible, dependable and composable services for an improved IoT ecosystem.