Many computational studies on hotspot microfluidic cooling devices found in the literature rely on simplified assumptions and conventions that do not capture the full complexity of the conjugate thermal problem, such as constant thermophysical fluid properties, radiation, and free air convection on the external walls. These assumptions are generally applied to typical microfluidic devices with a large number of microchannels and operating at Reynolds numbers between 100 and 1000. A one microchannel chip is a suitable starting point to analyze more systematically the implications of these assumptions, in particular at lower Reynolds numbers. Although it is a simpler system, it has been studied experimentally and numerically as a basic block of a thermal microfluidic device. In this work, we analyze the modeling of the overall heat transfer from a hotspot to a microfluidic heat sink, focusing on the effect of the different thermal transfer mechanisms (conduction, convection, and radiation), and temperature-dependent thermophysical properties of the fluid and the chip material. The study is developed as a function of the pressure difference applied to the system based on simulations performed using a finite volume method. Analyzing and comparing the different contributions to the energy losses, this work provides a critical discussion of the usually considered approximations, to make a reliable modeling of the overall thermal performance of a single rectangular straight channel embedded in a polydimethylsiloxane microfluidic chip.