We present the design and performance of QuEL-1 SE, which is a multichannel qubit controller developed for superconducting qubits. The system incorporates the active thermal stabilization of critical analog integrated circuits, such as phase-locked loops, amplifiers, and mixers, to suppress the long-term amplitude and phase drift. To evaluate the amplitude and phase stability, we simultaneously monitor 15 microwave output channels over 24 h using a common analog-to-digital converter. Across the channels, the normalized amplitude exhibits standard deviations of 0.09%-0.22% (mean: 0.15%), and the phase deviations are 0.35°-0.44° (mean: 0.39°). We further assess the impact of these deviations on quantum gate operations by estimating the average fidelity of an Xπ/2 gate under the coherent errors corresponding to the deviations. The resulting estimated contribution to the Xπ/2 gate infidelity due to amplitude noise (2 × 10-6) and phase misalignment (2 × 10-5) is significantly lower than typical fault-tolerance thresholds such as those of the surface [Fowler et al., Phys. Rev. A 86, 032324 (2012)]. These results demonstrate that the amplitude and phase stability of QuEL-1 SE enable reliable long-duration quantum operations, thus highlighting its utility as a scalable control platform for superconducting and other qubit modalities.