It's a fad. That said, the texture/sweetness of the pearls can vary a lot. (They're pretty finicky to cook correctly), so you are going to get a pretty big range of quality.
That said, I prefer egg custard if I get milk tea.
Well, the powdery stuff is quite similar to non-dairy creamer and thus is extremely delicious but also very very bad for your health, as they contain hydrogenated or even partially hydrogenated oils: thus may well contain trans fats!
personally, I always ask them to make it without any powders "no non-dairy creamers". in this case they'll use condensed milk or whole milk which is still bad for you but not as bad as trans fats.
That said, I prefer egg custard if I get milk tea.