I’m AuADHD, myself, but do not experience synthesisia so that certainly colors my perspective. People with sensory sensitivities would definitely be more prone to experiencing challenges with synthesisia and sensory overload but anyone can potentially be impacted under the right circumstances. Particularly intense sensory stimuli can be stressful. Make it multi-sensory and the intensity level is effectively increased.

Children who are otherwise neurotypical can sometimes have a little bit of a harder time with stress and intense sensations, which can manifest as meltdowns and the like.