It depends on what you consider the more demanding : ability mastering or being able to extract something valuable out of the merc overall.
On the first hand, you have mercs like Sparks and probably Fletcher which have indeed abilities that require some good aiming/timing skills to be used correctly. But once you get a grip on their abilities, everything sorts itself quite easily. On the other hand, you have mercs which have pretty simple abilities but are incredibly hard on the overall awareness and self positioning aspects, like Aura. Those can require massive experience to be truly efficient. Kira is in my opinion in between those two categories. Everything else, for various reasons, feel less demanding than those to me.
Nevertheless, light mercs will somehow have a tendency to have a higher skill ceiling as other mercs, as you directly start with a health handicap (which is clearly not compensated by their speed currently).