Sucralose, also known as "Splenda," is around 600 times sweeter than regular sugar, so your drinks can taste overwhelmingly sweet if you're not used to it.
The primary appeal of sucralose is that it has no calories, but this is because our bodies can't process it, meaning it leaves the body in the same form it entered.
Moreover, Starbucks' sugar-free syrups contain maltodextrin, an artificial carb that is notorious for causing blood sugar spikes, which can be potentially harmful for diabetics.